Sunday 8 December 2013

IPv6

IPv6 Tutorial

Internet Protocol version 6 (IPv6) is the latest revision of the Internet Protocol (IP) and the first version of the protocol to be widely deployed. IPv6 was developed by the Internet Engineering Task Force (IETF) to deal with the long-anticipated problem of IPv4 address exhaustion.

This tutorial will help you in understanding IPv6 and associated terminologies along with appropriate references and examples.

IPv6 - Overview

Internet Protocol version 6, is a new addressing protocol designed to incorporate whole sort of requirement of future internet known to us as Internet version 2. This protocol as its predecessor IPv4, works on Network Layer (Layer-3). Along with its offering of enormous amount of logical address space, this protocol has ample of features which addresses today’s shortcoming of IPv4.

Why new IP version?

So far, IPv4 has proven itself as a robust routable addressing protocol and has served human being for decades on its best-effort-delivery mechanism. It was designed in early 80’s and did not get any major change afterward. At the time of its birth, Internet was limited only to a few Universities for their research and to Department of Defense. IPv4 is 32 bits long which offers around 4,294,967,296 (2³²) addresses. This address space was considered more than enough that time. Given below are major points which played key role in birth of IPv6:

Internet has grown exponentially and the address space allowed by IPv4 is saturating. There is a requirement of protocol which can satisfy the need of future Internet addresses which are expected to grow in an unexpected manner.
Using features such as NAT, has made the Internet discontiguous i.e. one part which belongs to intranet, primarily uses private IP addresses; which has to go through number of mechanism to reach the other part, the Internet, which is on public IP addresses.
IPv4 on its own does not provide any security feature which is vulnerable as data on Internet, which is a public domain, is never safe. Data has to be encrypted with some other security application before being sent on Internet.
Data prioritization in IPv4 is not up to date. Though IPv4 has few bits reserved for Type of Service or Quality of Service, but they do not provide much functionality.
IPv4 enabled clients can be configured manually or they need some address configuration mechanism. There exists no technique which can configure a device to have globally unique IP address.

Why not IPv5?

Till date, Internet Protocol has been recognized has IPv4 only. Version 0 to 3 were used while the protocol was itself under development and experimental process. So, we can assume lots of background activities remain active before putting a protocol into production. Similarly, protocol version 5 was used while experimenting with stream protocol for internet. It is known to us as Internet Stream Protocol which used Internet Protocol number 5 to encapsulate its datagram. Though it was never brought into public use, but it was already used.

Here is a table of IP version and their use:

Brief History

After IPv4’s development in early 80s, the available IPv4 address pool begun to shrink rapidly as the demand of addresses exponentially increased with Internet. Taking pre-cognizance of situation that might arise IETF, in 1994, initiated the development of an addressing protocol to replace IPv4. The progress of IPv6 can be tracked by means of RFC published:

1998 – RFC 2460 – Basic Protocol
2003 – RFC 2553 – Basic Socket API
2003 – RFC 3315 – DHCPv6
2004 – RFC 3775 – Mobile IPv6
2004 – RFC 3697 – Flow Label Specification
2006 – RFC 4291 – Address architecture (revision)
2006 – RFC 4294 – Node requirement

June 06, 2012 some of Internet giants chose to put their Servers on IPv6. Presently they are using Dual Stack mechanism to implement IPv6 parallel with IPv4.

IPv6 - Features

The successor of IPv4 is not designed to be backward compatible. Trying to keep the basic functionalities of IP addressing, IPv6 is redesigned entirely. It offers the following features:

Larger Address Space:

In contrast to IPv4, IPv6 uses 4 times more bits to address a device on the Internet. This much of extra bits can provide approximately 3.4×1038 different combinations of addresses. This address can accumulate the aggressive requirement of address allotment for almost everything in this world. According to an estimate, 1564 addresses can be allocated to every square meter of this earth.
Simplified Header:

IPv6’s header has been simplified by moving all unnecessary information and options (which are present in IPv4 header) to the end of the IPv6 header. IPv6 header is only twice as bigger than IPv4 providing the fact the IPv6 address is four times longer.
End-to-end Connectivity:

Every system now has unique IP address and can traverse through the internet without using NAT or other translating components. After IPv6 is fully implemented, every host can directly reach other host on the Internet, with some limitations involved like Firewall, Organization’s policies, etc.
Auto-configuration:

IPv6 supports both stateful and stateless auto configuration mode of its host devices. This way absence of a DHCP server does not put halt on inter segment communication.
Faster Forwarding/Routing:

Simplified header puts all unnecessary information at the end of the header. All information in first part of the header are adequate for a Router to take routing decision thus making routing decision as quickly as looking at the mandatory header.
IPSec:

Initially it was decided for IPv6 to must have IPSec security, making it more secure than IPv4. This feature has now been made optional.
No Broadcast:

Though Ethernet/Token Ring are considered as broadcast network because they support Broadcasting, IPv6 does not have any Broadcast support anymore left with it. It uses multicast to communicate with multiple hosts.
Anycast Support:

This is another characteristic of IPv6. IPv6 has introduced Anycast mode of packet routing. In this mode, multiple interfaces over the Internet are assigned same Anycast IP address. Routers, while routing, sends the packet to the nearest destination.
Mobility:

IPv6 was designed keeping mobility feature in mind. This feature enables hosts (such as mobile phone) to roam around in different geographical area and remain connected with same IP address. IPv6 mobility feature takes advantage of auto IP configuration and Extension headers.
Enhanced Priority support:

Where IPv4 used 6 bits DSCP (Differential Service Code Point) and 2 bits ECN (Explicit Congestion Notification) to provide Quality of Service but it could only be used if the end-to-end devices support it, that is, the source and destination device and underlying network must support it.

In IPv6, Traffic class and Flow label are used to tell underlying routers how to efficiently process the packet and route it.
Smooth Transition:

Large IP address scheme in IPv6 enables to allocate devices with globally unique IP addresses. This assures that mechanism to save IP addresses such as NAT is not required. So devices can send/receive data between each other, for example VoIP and/or any streaming media can be used much efficiently.

Other fact is, the header is less loaded so routers can make forwarding decision and forward them as quickly as they arrive.
Extensibility:

One of the major advantage of IPv6 header is that it is extensible to add more information in the option part. IPv4 provides only 40-bytes for options whereas options in IPv6 can be as much as the size of IPv6 packet itself.

IPv6 - Addressing Modes

In computer networking, addressing mode refers to the mechanism how we address a host on the network. IPv6 offers several types of modes by which a single host can be addressed, more than one host can be addressed at once or the host at closest distance can be addressed.

Unicast

In unicast mode of addressing, an IPv6 interface (host) is uniquely identified in a network segment. The IPv6 packet contains both source and destination IP addresses. A host interface is equipped with an IP address which is unique in that network segment. A network switch or router when receives a unicast IP packet, destined to single host, sends out to one of its outgoing interface which connects to that particular host.

Multicast

The IPv6 multicast mode is same as that of IPv4. The packet destined to multiple hosts is sent on a special multicast address. All hosts interested in that multicast information, need to join that multicast group first. All interfaces which have joined the group receive the multicast packet and process it, while other hosts not interested in multicast packets ignore the multicast information.

Anycast

IPv6 has introduced a new type of addressing, which is called Anycast addressing. In this addressing mode, multiple interfaces (hosts) are assigned same Anycast IP address. When a host wishes to communicate with a host equipped with an Anycast IP address, sends a Unicast message. With the help of complex routing mechanism, that Unicast message is delivered to the host closest to the Sender, in terms of Routing cost.

Let’s take an example of TutorialPoints.com Web Servers, located in all continents. Assume that all Web Servers are assigned single IPv6 Anycast IP Address. Now when a user from Europe wants to reach only4programmers.blogspot.com the DNS points to the server which is physically located in Europe itself. If a user from India tries to reach only4programmers.blogspot.com, the DNS will then point to Web Server physically located in Asia only. Nearest or Closest terms are used in terms of Routing Cost.

In the above picture, When a client computer tries to reach a Server, the request is forwarded to the Server with lowest Routing Cost.

IPv6 - Address Types & Formats

Hexadecimal Number System

Before introducing IPv6 Address format, we shall look into Hexadecimal Number System. Hexadecimal is positional number system which uses radix (base) of 16. To represent the values in readable format, this system uses 0-9 symbols to represent values from zero to nine and A-F symbol to represent values from ten to fifteen. Every digit in Hexadecimal can represent values from 0 to 15.

Address Structure

An IPv6 address is made of 128 bits divided into eight 16-bits blocks. Each block is then converted into 4-digit Hexadecimal numbers separated by colon symbol.

For example, the below is 128 bit IPv6 address represented in binary format and divided into eight 16-bits blocks:

0010000000000001 0000000000000000 0011001000110100 1101111111100001 0000000001100011 0000000000000000 0000000000000000 1111111011111011

Each block is then converted into Hexadecimal and separated by ‘:’ symbol:

2001:0000:3238:DFE1:0063:0000:0000:FEFB

Even after converting into Hexadecimal format, IPv6 address remains long. IPv6 provides some rules to shorten the address. These rules are:

Rule:1 Discard leading Zero(es):

In Block 5, 0063, the leading two 0s can be omitted, such as (5th block):

2001:0000:3238:DFE1:63:0000:0000:FEFB

Rule:2 If two of more blocks contains consecutive zeroes, omit them all and replace with double colon sign ::, such as (6th and 7th block):

2001:0000:3238:DFE1:63::FEFB

Consecutive blocks of zeroes can be replaced only once by :: so if there are still blocks of zeroes in the address they can be shrink down to single zero, such as (2nd block):

2001:0:3238:DFE1:63::FEFB

Interface ID

IPv6 has three different type of Unicast Address scheme. The second half of the address (last 64 bits) is always used for Interface ID. MAC address of a system is composed of 48-bits and represented in Hexadecimal. MAC address is considered to be uniquely assigned worldwide. Interface ID takes advantage of this uniqueness of MAC addresses. A host can auto-configure its Interface ID by using IEEE’s Extended Unique Identifier (EUI-64) format. First, a Host divides its own MAC address into two 24-bits halves. Then 16-bit Hex value 0xFFFE is sandwiched into those two halves of MAC address, resulting in 64-bit Interface ID.

Global Unicast Address

This address type is equivalent to IPv4’s public address. Global Unicast addresses in IPv6 are globally identifiable and uniquely addressable.

Global Routing Prefix: The most significant 48-bits are designated as Global Routing Prefix which is assigned to specific Autonomous System. Three most significant bits of Global Routing Prefix is always set to 001.

Link-Local Address

Auto-configured IPv6 address is known as Link-Local address. This address always starts with FE80. First 16 bits of Link-Local address is always set to 1111 1110 1000 0000 (FE80). Next 48-bits are set to 0, thus:

Link-Local addresses are used for communication among IPv6 hosts on a link (broadcast segment) only. These addresses are not routable so a Router never forwards these addresses outside the link.

Unique-Local Address

This type of IPv6 address which is though globally unique, but it should be used in local communication. This address has second half of Interface ID and first half is divided among Prefix, Local Bit, Global ID and Subnet ID.

Prefix is always set to 1111 110. L bit, which is set to 1 if the address is locally assigned. So far the meaning of L bit to 0 is not defined. Therefore, Unique Local IPv6 address always starts with ‘FD’.

SCOPE OF IPV6 UNICAST ADDRESSES:

The scope of Link-local address is limited to the segment. Unique Local Address are though locally global but are not routed over the Internet, limiting their scope to an organization’s boundary. Global Unicast addresses are globally unique and recognizable. They shall make the essence of Internet v2 addressing.

IPv6 - Special Addresses

Version 6 has slightly complex structure of IP address than that of IPv4. IPv6 has reserved few addresses and address notations for special purposes. See the table below:

Special Addresses:

As shown in the table above 0:0:0:0:0:0:0:0/128 address does not specify to anything and is said to be an unspecified address. After simplifying, all 0s are compacted to ::/128.
In IPv4, address 0.0.0.0 with netmask 0.0.0.0 represents default route. The same concept is also applie to IPv6, address 0:0:0:0:0:0:0:0 with netmask all 0s represents default route. After applying IPv6 simplying rule this address is compressed to ::/0.
Loopback addresses in IPv4 are represented by 127.0.0.1 to 127.255.255.255 series. But in IPv6, only 0:0:0:0:0:0:0:1/128 address represents Loopback address. After simplying loopback address, it can be represented as ::1/128.

Reserved Multicast Address for Routing Protocols:

The above table shows reserved multicast addresses used by interior routing protocol.
All addresses are reserved in similar IPv4 fashion

Reserved Multicast Address for Routers/Node:

These addresses helps routers and hosts to speak to available routers and hosts on a segment without being configured with an IPv6 address. Hosts use EUI-64 based auto-configuration to self-configure an IPv6 address and then speaks to available hosts/routers on the segment by means of these addresses.

IPv6 - Headers

The wonder of IPv6 lies in its header. IPv6 address is 4 times larger than IPv4 but the IPv6 header is only 2 times larger than that of IPv4. IPv6 headers have one Fixed Header and zero or more Optional (Extension) Headers. All necessary information which is essential for a router is kept in Fixed Header. Extension Header contains optional information which helps routers to understand how to handle a packet/flow.

Fixed Header

IPv6 fixed header is 40 bytes long and contains the following information.

S.N.	Field & Description
1	Version (4-bits): This represents the version of Internet Protocol, i.e. 0110.
2	Traffic Class (8-bits): These 8 bits are divided into two parts. Most significant 6 bits are used for Type of Service, which tells the Router what services should be provided to this packet. Least significant 2 bits are used for Explicit Congestion Notification (ECN).
3	Flow Label (20-bits): This label is used to maintain the sequential flow of the packets belonging to a communication. The source labels the sequence which helps the router to identify that this packet belongs to a specific flow of information. This field helps to avoid re-ordering of data packets. It is designed for streaming/real-time media.
4	Payload Length (16-bits): This field is used to tell the routers how much information this packet contains in its payload. Payload is composed of Extension Headers and Upper Layer data. With 16 bits, up to 65535 bytes can be indicated but if Extension Headers contain Hop-by-Hop Extension Header than payload may exceed 65535 bytes and this field is set to 0.
5	Next Header (8-bits): This field is used to indicate either the type of Extension Header, or if Extension Header is not present then it indicates the Upper Layer PDU. The values for the type of Upper Layer PDU is same as IPv4’s.
6	Hop Limit (8-bits): This field is used to stop packet to loop in the network infinitely. This is same as TTL in IPv4. The value of Hop Limit field is decremented by 1 as it passes a link (router/hop). When the field reaches 0 the packet is discarded.
7	Source Address (128-bits): This field indicates the address of originator of the packet.
8	Destination Address (128-bits): This field provides the address of intended recipient of the packet.

Extension Headers

In IPv6, the Fixed Header contains only information which is necessary and avoiding information which is either not required or is rarely used. All such information, is put between the Fixed Header and Upper layer header in the form of Extension Headers. Each Extension Header is identified by a distinct value.

When Extension Headers are used, IPv6 Fixed Header’s Next Header field points to the first Extension Header. If there is one more Extension Header, then first Extension Header’s ‘Next-Header’ field point to the second one, and so on. The last Extension Header’s ‘Next-Header’ field point to Upper Layer Header. Thus all headers from point to the next one in a linked list manner.

If the Next Header field contains value 59, it indicates that there’s no header after this header, not even Upper Layer Header.

The following Extension Headers must be supported as per RFC 2460:

The sequence of Extension Headers should be:

These headers:

1. Should be processed by First and subsequent destinations.
2. Should be processed by Final Destination.

Extension Headers are arranged one after another in a Linked list manner, as depicted in the diagram below:

[*Image: Extension Headers Connected Format*]

IPv6 - Subnetting

In IPv4, addresses were created in classes. Classful IPv4 addresses clearly defines the bits used for network prefixes and the bits used for hosts on that network. To subnet in IPv4 we play with the default classful netmask which allows us to borrow hosts bit to be used as subnet bits. This results in multiple subnets but less hosts per subnet. That is, when we borrow host bit to create a subnet that costs us in lesser bit to be used for host addresses.

IPv6 addresses uses 128 bits to represent an address which includes bits to be used for subnetting. Second half of the address (least significant 64 bits) is always used for Hosts only. Therefore, there is no compromise if we subnet the network.

16 Bits of subnet is equivalent to IPv4’s Class B Network. Using these subnet bits an organization can have more 65 thousands of subnets which is by far, more than enough.

Thus routing prefix is /64 and host portion is 64 bits. We though, can further subnet the network beyond 16 bits of Subnet ID, borrowing hosts bit but it is recommended that 64 bits should always be used for hosts addresses because auto-configuration requires 64 bits.

IPv6 subnetting works on the same concept as Variable Length Subnet Masking in IPv4.

/48 prefix can be allocated to an organization providing it the benefit of having up to /64 subnet prefixes, which is 65535 sub-networks, each having 264 hosts. A /64 prefix can be assigned to a point-to-point connection where there are only two hosts (or IPv6 enabled devices) on a link.

Transition From IPv4 to IPv6

One problem in transition from IPv4 to IPv6 completely is that IPv6 is not backward compatible. This results in a situation where either a site is on IPv6 or it is not. Unlike an implementation of new technology where the newer one is backward compatible so the older system can still work with the newer without any additional changes.

To overcome this short-coming, there exist few technologies which can be used in slow and smooth transition from IPv4 to IPv6:

Dual Stack Routers

A router can be installed with both IPv4 and IPv6 addresses configured on its interfaces pointing to the network of relevant IP scheme.

In above diagram, a Server which is having IPv4 as well as IPv6 address configured for it now can speak with all hosts on IPv4 network and IPv6 network with help of Dual Stack Router. Dual Stack Router, can communicate with both networks and provides a medium for hosts to access Server without changing their respective IP version.

Tunneling

In a scenario where different IP versions exist on intermediate path or transit network, tunneling provides a better solution where user’s data can pass through a non-supported IP version.

The above diagram depicts how two remote IPv4 networks can communicate via Tunnel, where the transit network was on IPv6. Vice versa is also possible where transit network is on IPv6 and remote sites which intends to communicate, are on IPv4.

NAT Protocol Translation

This is another important method of transition to IPv6 by means of a NAT-PT (Network Address Translation – Protocol Translation) enabled device. With help of NAT-PT device, actual conversion happens between IPv4 and IPv6 packets and vice versa. See the diagram below:

A host with IPv4 address sends a request to IPv6 enabled Server on Internet which does not understand IPv4 address. In this scenario, NAT-PT device can help them communicate. When IPv4 host sends a request packet to IPv6 Server, NAT-PT device/router, strips down the IPv4 packet, removes IPv4 header and adds IPv6 header and passes it through the Internet. When a response from IPv6 Server comes for IPv4 host, the router does vice versa.

IPv6 Mobility

When a host is connected to one link or network, it acquires an IP address and all communication happens using that IP address on that link. As soon as, the same host changes its physical location, that is, moves into some different area / subnet / network / link, its IP address changes accordingly and all communication happening on the host using old IP address, goes down.

IPv6 mobility provides a mechanism which equips a host with an ability to roam around among different links without losing any communication/connection and its IP address.

Multiple entities are involved in this technology:

Mobile Node: The device which needs IPv6 mobility.
Home Link: This link is configured with the home subnet prefix and this is where the Mobile IPv6 device gets its Home Address.
Home Address: This is the address which Mobile Node acquires from Home Link. This is permanent address of Mobile Node. If the Mobile Node remains in the same Home Link, the communication among various entities happens as usual.
Home Agent: This is a router which acts as registrar for Mobile Nodes. Home Agent is connected to Home Link and maintains information about all Mobile Nodes, their Home Addresses and their present IP addresses.
Foreign Link: Any other Link which is not Mobile Node’s Home Link.
Care-of Address: When a Mobile Node attaches to a Foreign Link, it acquires a new IP address of that Foreign Link’s subnet. Home Agent maintains the information of both Home Address and Care-of Address. Multiple Care-of addresses can be assigned to Mobile Node, but at any instance only one Care-of Address has binding with Home Address.
Correspondent Node: Any IPv6 enable device which intends to have communication with Mobile Node.

Mobility Operation

When Mobile Node stays in its Home Link, all communications happen on its Home Address. As shown below:

[*Image: Mobile Node connected to Home Link*]

When Mobile Node leaves its Home Link and is connected to some Foreign Link, the Mobility feature of IPv6 comes into play. After connecting to Foreign Link, Mobile Node acquires an IPv6 address from Foreign Link. This address is called Care-of Address. Mobile Node sends binding request to its Home Agent with the new Care-of Address. Home Agent binds Mobile Node’s Home Address with Care-of Address, establishing a Tunnel between both.

Whenever a Correspondent Node tries to establish connection with Mobile Node (on its Home Address), the Home Agent intercepts the packet and forwards to Mobile Node’s Care-of Address over the Tunnel which was already established.

[*Image: Mobile Node connected to Foreign Link*]

Route Optimization

When a Correspondent Node initiate communication by sending packets to Mobile Node on Home Address, these packets are tunneled to Mobile Node by Home Agent. In Route Optimization mode, when the Mobile Node receives packet from Correspondent Node, it does not forward replies to Home Agent. Rather it sends its packet directly to Correspondent Node using Home Address as Source Address. This mode is optional and not used by default.

IPv6 Routing

Routing concepts remain same in case of IPv6 but almost all routing protocol have been redefined accordingly. We have seen in Communication in IPv6 segment, how a host speaks to its gateway. Routing is a process to forward routable data choosing best route among several available routes or path to the destination. A router is a device which forwards data which is not explicitly destined to it.

There exists two forms of routing protocols

Distance Vector Routing Protocol: A router running distance vector protocol advertises its connected routes and learns new routes from its neighbors. The routing cost to reach a destination is calculated by means of hops between the source and destination. A Router generally relies on its neighbor for best path selection, also known as “routing-by-rumors”. RIP and BGP are Distance Vector Protocols.
Link-State Routing Protocol: This protocol acknowledges the state of a Link and advertises to its neighbors. Information about new links is learnt from peer routers. After all the routing information has been converged, Link-State Routing Protocol uses its own algorithm to calculate best path to all available links. OSPF and IS-IS are link state routing protocols and both uses Djikstra’s Shortest Path First algorithm.

Routing protocols can be divided in two categories:

Interior Routing Protocol: Protocols in this categories are used within an Autonomous System or organization to distribute routes among all routers inside its boundary. Examples: RIP, OSPF.
Exterior Routing Protocol: Whereas an Exterior Routing Protocol distributes routing information between two different Autonomous Systems or organization. Examples: BGP.

Routing protocols

RIPng

RIPng stands for Routing Information Protocol Next Generation. This is an Interior Routing Protocol and is a Distance Vector Protocol. RIPng has been upgraded to support IPv6.
OSPFv3

Open Shortest Path First version 3 is an Interior Routing Protocol which is modified to support IPv6. This is a Link-State Protocol and uses Djikrasta’s Shortest Path First algorithm to calculate best path to all destinations.

BGPv4

BGP stands for Border Gateway Protocol. It is the only open standard Exterior Gateway Protocol available. BGP is a Distance Vector protocol which takes Autonomous System as calculation metric, instead of number of routers as Hop. BGPv4 is an upgrade of BGP to support IPv6 routing.

Protocols changed to support IPv6:

ICMPv6: Internet Control Message Protocol version 6 is an upgraded implementation of ICMP to accommodate IPv6 requirements. This protocol is used for diagnostic functions, error and information message, statistical purposes. ICMPv6’s Neighbor Discovery Protocol replaces ARP and helps discover neighbor and routers on the link.
DHCPv6: Dynamic Host Configuration Protocol version 6 is an implementation of DHCP. Though IPv6 enabled hosts do not require any DHCPv6 Server to acquire IP address as they can be auto-configured. Neither do they need DHCPv6 to locate DNS server because DNS can be discovered and configured via ICMPv6 Neighbor Discovery Protocol. Yet DHCPv6 Server can be used to provide these information.
DNS: There has been no new version of DNS but it is now equipped with extensions to provide support for querying IPv6 addresses. A new AAAA (quad-A) record has been added to reply IPv6 query messages. Now DNS can reply with both IP versions (4 & 6) without any change in query format.

IPv6 Summary

IPv4 since 1982, has been an undisputed leader of Internet. With IPv4’s address space exhaustion IPv6 is now taking over the control of Internet, which is called Internet2.

IPv4 is widely deployed and migration to IPv6 would not be easy. So far IPv6 could penetrate IPv4’s address space by less than 1%.

The world has celebrated ‘World IPv6 Day’ on June 08, 2011 with a purpose to test IPv6 address over Internet in full. On June 06, 2012 the Internet community officially launched IPv6. This day all ISPs who were offering IPv6 were to enable it on public domain and were to keep it enable. All the device manufacturer also participated to offer IPv6 by-default enabled on devices.

This was a step towards encouraging Internet community to migrate to IPv6.

Organizations are provided plenty of ways to migrate from IPv4 to IPv6. Also organization, willing to test IPv6 before migrating completely can run both IPv4 and IPv6 simultaneously. Networks of different IP versions can communicate and user data can be tunneled to walk to the other side.

Future of IPv6

IPv6 enabled Internet version 2 will replace todays IPv4 enabled Internet. When Internet was launched with IPv4, developed countries like US and Europe took the larger space of IPv4 for deployment of Internet in their respective countries keeping future need in mind. But Internet exploded everywhere reaching and connecting every country of the world increasing the requirement of IPv4 address space. As a result, till this day US and Europe have many IPv4 address space left with them and countries like India and China are bound to address their IP space requirement by means of deployment of IPv6.

Most of the IPv6 deployment is being done outside US, Europe. India and China are moving forward to change their entire space to IPv6. China has announced a five year deployment plan named China Next Generation Internet.

After June 06, 2012 all major ISPs were shifted to IPv6 and rest of them are still moving.

IPv6 provides ample of address space and is designed to expand today’s Internet services. Feature-rich IPv6 enabled Internet version 2 may deliver more than expected.

Saturday 7 December 2013

IPv4

IPv4 Tutorial

Internet Protocol version 4 (IPv4) is the fourth version in the development of the Internet Protocol (IP) and the first version of the protocol to be widely deployed. IPv4 is described in IETF publication RFC 791 (September 1981), replacing an earlier definition (RFC 760, January 1980).

This tutorial will help you in understanding IPv4 and associated terminologies along with appropriate references and examples.

IPv4 - Overview

This era is said to be the era of computers. Computers have significantly changed lives and the way we used to live. A computing device when connected to other computing device(s) enables us to share data and information at lightning fast speed.

What is Network?

A Network in the world of computers is said to be a collection of interconnected hosts, via some shared media which can be wired or wireless. A computer network enables its hosts to share and exchange data and information over the media. Network can be a Local Area Network spanned across an office or Metro Area Network spanned across a city or Wide Area Network which can be spanned across cities and provinces.

Computer network can be as simple as two PCs connected together via a single copper cable or it can be grown up to the complexity where every computer in this world is connected to every other, the Internet. A network then includes more and more components to reach its ultimate goal of data exchange. Below is a brief description of the components involved in computer network:

Hosts - Hosts are said to be situated at ultimate end of the network, i.e. a host is a source of information and another host will be the destination. Information flows end to end between hosts. A host can be a user’s PC, an internet Server, a database server etc.
Media - If wired, then it can be copper cable, fiber optic cable, coaxial cable or if wireless, it can be free-to-air radio frequency or some special wireless band. Wireless frequencies can be used to interconnect remote sites too.
Hub - A hub is a multiport repeater and it is used to connect hosts in a LAN segment. Because of low throughputs hubs are now rarely used. Hub works on Layer-1 (Physical Layer) of OSI Model.
Switch - A Switch is a multiport bridge and is used to connect hosts in a LAN segment. Switches are much faster than Hubs and operate on wire speed. Switch works on Layer-2 (Data Link Layer) but Layer-3 (Network Layer) switches are also available.
Router - A router is Layer-3 (Network Layer) device which makes routing decisions for the data/information sent for some remote destination. Routers make the core of any interconnected network and the Internet.
Gateways - A software or combination of software and hardware putting together works for exchanging data among networks which are using different protocols for sharing data.
Firewall - Software or combination of software and hardware, used to protect users’ data from unintended recipients on the network/internet.

All components in a network ultimately serve the hosts.

Host Addressing

Communication between hosts can happen only if they can identify each other on the network. In a single collision domain (where every packet sent on the segment by one host is heard by every other host) hosts can communicate directly via MAC address.

MAC address is a factory coded 48-bits hardware address which can also uniquely identify a host in the world. But if a host wants to communicate with a remote host, i.e. not in the same segment or logically not connected, then some means of addressing is required to identify the remote host uniquely. A logical address is given to all hosts connected to Internet and this logical address is calledInternet Protocol Address.

IPv4 - OSI Model

International Standard Organization has a well-defined Model for Communication Systems known as Open System Interconnection, or OSI Model. This layered model is a conceptualized view of how one system should communicate with the other, using various protocols defined in each layer. Further, each layer is designated to a well-defined part of communication system. For example, the Physical layer defines all the components of physical nature, i.e. wires, frequencies, pulse codes, voltage transmission etc. of a communication system.

OSI Model has following seven layers:

Application Layer (Layer-7): This is where the user application sits who needs to transfer data between or among hosts. For example: HTTP, file transfer application (FTP) and electronic mail etc.
Presentation Layer (Layer-6): This layer helps to understand data representation in one form on a host to other host in their native representation. Data from the sender is converted to on-the-wire data (general standard format) and at the receiver’s end it is converted to the native representation of the receiver.
Session Layer (Layer-5): This layer provides session management capabilities between hosts. For example if some host needs a password verification for access and if credentials are provided then for that session password verification does not happen again. This layer can assist in synchronization, dialog control and critical operation management (e.g., an online bank transaction).
Transport Layer (Layer-4): This layer provides end to end data delivery between/among hosts. This layer takes data from above layer and breaks it into smaller units called Segments and then gives it to Network layer for transmission.
Network Layer (Layer-3): This layer helps to uniquely identify hosts beyond the subnets and defines the path which the packets will follow or be routed to reach the destination.
Data Link Layer (Layer-2): This layer takes the raw transmission data (signal, pulses etc.) from Physical Layer and makes Data Frames and sends that to upper layer and vice versa. This layer also checks any transmission errors and sort it out accordingly.
Physical Layer (Layer-1): This layer deals with hardware technology and actual communication mechanism like signaling, voltage, cable type and length etc.

Network Layer

The network layer is responsible for carrying data from one host to another. It provides means to allocate logical addresses to hosts and identify them uniquely using the same. Network layer takes data units from Transport Layer and cuts them in to smaller unit called Data Packet.

Network layer defines the data path, the packets should follow to reach the destination. Routers work on this layer and provides mechanism to route data to its destination.

IPv4 - TCP/IP Model

Majority of the internet uses a protocol suite called the Internet Protocol Suite also known as TCP/IP protocol suite. This suite is a combination of protocols which encompasses a number of different protocols for different purpose and need. Because the two major protocols in this suites are TCP (Transmission Control Protocol) and IP (Internet Protocol), this is commonly termed as TCP/IP Protocol suite. This protocol suite has its own reference model which it follows over the internet. In contrast with OSI model, this model of protocols contains less layers.

[ Comparative depiction of OSI and TCP/IP Reference Models ]

This model is indifferent to the actual hardware implementation, i.e. Physical layer of OSI Model. This is why this model can be implemented on almost all underlying technologies. Transport and Internet layers correspond to the same peer layers. All three top layers of OSI Model are compressed together in single Application layer of TCP/IP Model.

Internet Protocol Version 4 (IPv4)

Internet Protocol is one of the major protocol in TCP/IP protocols suite. This protocol works at Network layer of OSI model and at Internet layer of TCP/IP model. Thus this protocol has the responsibility of identification of hosts based upon their logical addresses and to route data between/among them over the underlying network.

IP provides a mechanism to uniquely identify host by IP addressing scheme. IP uses best effort delivery, i.e. it does not guarantee that packets would be delivered to destined host but it will do its best to reach the destination. Internet Protocol version 4 uses 32-bit logical address.

IPv4 - Packet Structure

Internet Protocol being a layer-3 protocol (OSI) takes data Segments from layer-4 (Transport) and divides it into what’s called packet. IP packet encapsulates data unit received from above layer and adds its own header information.

The encapsulated data is referred to as IP Payload. IP header contains all the necessary information to deliver the packet at the other end.

IP header includes many relevant information including Version Number, which, in this context, is 4. Other details are as follows:

Version: Version no. of Internet Protocol used (e.g. IPv4)
IHL: Internet Header Length, Length of entire IP header
DSCP: Differentiated Services Code Point, This is Type of Service.
ECN: Explicit Congestion Notification, carries information about the congestion seen in the route.
Total Length: Length of entire IP Packet (including IP header and IP Payload)
Identification: If IP packet is fragmented during the transmission, all the fragments contain same identification no. to identify original IP packet they belong to.
Flags: As required by the network resources, if IP Packet is too large to handle these ‘flags’ tell that if they can be fragmented or not. In this 3-bit flag, the MSB is always set to ‘0’.
Fragment Offset: This offset tells the exact position of the fragment in the original IP Packet.
Time to Live: To avoid looping in the network, every packet is sent with some TTL value set, which tells the network how many routers (hops) this packet can cross. At each hop, its value is decremented by one and when the value reaches zero, the packet is discarded.
Protocol: Tells the Network layer at the destination host, to which Protocol this packet belongs to, i.e. the next level Protocol. For example protocol number of ICMP is 1, TCP is 6 and UDP is 17.
Header Checksum: This field is used to keep checksum value of entire header which is then used to check if the packet is received error-free.
Source Address: 32-bit address of the Sender (or source) of the packet.
Destination Address: 32-bit address of the Receiver (or destination) of the packet.
Options: This is optional field, which is used if the value of IHL is greater than 5. These options may contain values for options such as Security, Record Route, Time Stamp etc.

IPv4 - Addressing

IPv4 supports three different type of addressing modes:

Unicast Addressing Mode:

In this mode, data is sent only to one destined host. The Destination Address field contains 32- bit IP address of the destination host. Here client sends data to the targeted server:

Broadcast Addressing Mode:

In this mode the packet is addressed to all hosts in a network segment. The Destination Address field contains special broadcast address i.e. 255.255.255.255. When a host sees this packet on the network, it is bound to process it. Here client sends packet, which is entertained by all the Servers:

Multicast Addressing Mode:

This mode is a mix of previous two modes, i.e. the packet sent is neither destined to a single host nor all the host on the segment. In this packet, the Destination Address contains special address which starts with 224.x.x.x and can be entertained by more than one host.

Here a server sends packets which are entertained by more than one Servers. Every network has one IP address reserved for network number which represents the network and one IP address reserved for Broadcast Address, which represents all the host in that network.

Hierarchical Addressing Scheme

IPv4 uses hierarchical addressing scheme. An IP address which is 32-bits in length, is divided into two or three parts as depicted:

A single IP address can contain information about the network and its sub-network and ultimately the host. This scheme enables IP Address to be hierarchical where a network can have many sub-networks which in turn can have many hosts.

Subnet Mask

The 32-bit IP address contains information about the host and its network. It is very necessary to distinguish the both. For this, routers use Subnet Mask, which is as long as the size of the network address in the IP address. Subnet Mask is also 32 bits long. If the IP address in binary is ANDed with its Subnet Mask, the result yields the Network address. For example, say the IP Address 192.168.1.152 and the Subnet Mask is 255.255.255.0 then

This way Subnet Mast helps extract Network ID and Host from an IP Address. It can be identified now that 192.168.1.0 is the Network number and 192.168.1.152 is the host on that network.

Binary Representation

The positional value method is the simplest form of converting binary from decimal value. IP address is 32 bit value which is divided into 4 octets. A binary octet contains 8 bits and the value of each bit can be determined by the position of bit value '1' in the octet.

Positional value of bits is determined by 2 raised to power (position – 1), that is the value of a bit 1 at position 6 is 26-1 that is 25 that is 32. The total value of the octet is determined by adding up the positional value of bits. The value of 11000000 is 128+64 = 192. Some Examples are shown in the table below:

IPv4 - Address Classes

Internet Protocol hierarchy contains several classes of IP Addresses to be used efficiently in various situation as per the requirement of hosts per network. Broadly, IPv4 Addressing system is divided into 5 classes of IP Addresses. All the 5 classes are identified by the first octet of IP Address.

Internet Corporation for Assigned Names and Numbers - responsible for assigning IP addresses.

The first octet referred here is the left most of all. The octets numbered as follows depicting dotted decimal notation of IP Address:

Number of networks and number of hosts per class can be derived by this formula:

When calculating hosts IP addresses, 2 IP addresses are decreased because they cannot be assigned to hosts i.e. the first IP of a network is network number and the last IP is reserved for Broadcast IP.

Class A Address

The first bit of the first octet is always set to 0 (zero). Thus the first octet ranges from 1 – 127, i.e.

Class A addresses only include IP starting from 1.x.x.x to 126.x.x.x only. The IP range 127.x.x.x is reserved for loopback IP addresses.

The default subnet mask for Class A IP address is 255.0.0.0 which implies that Class A addressing can have 126 networks (2⁷-2) and 16777214 hosts (2²⁴-2).

Class A IP address format thus, is 0NNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH

Class B Address

An IP address which belongs to class B has the first two bits in the first octet set to 10, i.e.

Class B IP Addresses range from 128.0.x.x to 191.255.x.x. The default subnet mask for Class B is 255.255.x.x.

Class B has 16384 (2¹⁴) Network addresses and 65534 (2¹⁶-2) Host addresses.

Class B IP address format is, 10NNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH

Class C Address

The first octet of Class C IP address has its first 3 bits set to 110, that is

Class C IP addresses range from 192.0.0.x to 192.255.255.x. The default subnet mask for Class B is 255.255.255.x.

Class C gives 2097152 (2²¹) Network addresses and 254 (2⁸-2) Host addresses.

Class C IP address format is 110NNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

Class D Address

Very first four bits of the first octet in Class D IP addresses are set to 1110, giving a range of

Class D has IP address rage from 224.0.0.0 to 239.255.255.255. Class D is reserved for Multicasting. In multicasting data is not destined for a particular host, that's why there is no need to extract host address from the IP address, and Class D does not have any subnet mask.

Class E Address

This IP Class is reserved for experimental purposes only like for R&D or Study. IP addresses in this class ranges from 240.0.0.0 to 255.255.255.254. Like Class D, this class too is not equipped with any subnet mask.

IPv4 - Subnetting (CIDR)

Each IP class is equipped with its own default subnet mask which bounds that IP class to have prefixed number of Networks and prefixed number of Hosts per network. Classful IP addressing does not provide any flexibility of having less number of Hosts per Network or more Networks per IP Class.

CIDR or Classless Inter Domain Routing provides the flexibility of borrowing bits of Host part of the IP address and using them as Network in Network, called Subnet. By using subnetting, one single Class A IP addresses can be used to have smaller sub-networks which provides better network management capabilities.

Class A Subnets

In Class A, only the first octet is used as Network identifier and rest of three octets are used to be assigned to Hosts (i.e. 16777214 Hosts per Network). To make more subnet in Class A, bits from Host part are borrowed and the subnet mask is changed accordingly.

For example, if one MSB (Most Significant Bit) is borrowed from host bits of second octet and added to Network address, it creates two Subnets (2¹=2) with (2²³-2) 8388606 Hosts per Subnet.

The Subnet mask is changed accordingly to reflect subnetting. Given below is a list of all possible combination of Class A subnets:

In case of subnetting too, the very first and last IP address of every subnet is used for Subnet Number and Subnet Broadcast IP address respectively. Because these two IP addresses cannot be assigned to hosts, Sub-netting cannot be implemented by using more than 30 bits as Network Bits which provides less than two hosts per subnet.

Class B Subnets

By Default, using Classful Networking, 14 bits are used as Network bits providing (2¹⁴) 16384 Networks and (2¹⁶-1) 65534 Hosts. Class B IP Addresses can be subnetted the same way as Class A addresses, by borrowing bits from Host bits. Below is given all possible combination of Class B subnetting:

Class C Subnets

Class C IP addresses normally assigned to a very small size network because it only can have 254 hosts in a network. Given below is a list of all possible combination of subnetted Class B IP address:

IPv4 - Variable Length Subnet Masking (VLSM)

Internet Service Providers may face a situation where they need to allocate IP subnets of different sizes as per the requirement of customer. One customer may ask Class C subnet of 3 IP addresses and another may ask for 10 IPs. For an ISP, it is not feasible to divide the IP addresses into fixed size subnets, rather he may want to subnet the subnets in such a way which results in minimum wastage of IP addresses.

For example, an administrator have 192.168.1.0/24 network. The suffix /24 (pronounced as "slash 24") tells the number of bits used for network address. He is having three different departments with different number of hosts. Sales department has 100 computers, Purchase department has 50 computers, Accounts has 25 computers and Management has 5 computers. In CIDR, the subnets are of fixed size. Using the same methodology the administrator cannot fulfill all the requirements of the network.

The following procedure shows how VLSM can be used in order to allocate department-wise IP addresses as mentioned in the example.

Step - 1

Make a list of Subnets possible.

Step - 2

Sort the requirements of IPs in descending order (Highest to Lowest).

Sales 100
Purchase 50
Accounts 25
Management 5

Step - 3

Allocate the highest range of IPs to the highest requirement, so let's assign 192.168.1.0 /25 (255.255.255.128) to Sales department. This IP subnet with Network number 192.168.1.0 has 126 valid Host IP addresses which satisfy the requirement of Sales Department. The subnet Mask used for this subnet has 10000000 as the last octet.

Step - 4

Allocate the next highest range, so let's assign 192.168.1.128 /26 (255.255.255.192) to Purchase department. This IP subnet with Network number 192.168.1.128 has 62 valid Host IP Addresses which can be easily assigned to all Purchase department's PCs. The subnet mask used has 11000000 in the last octet.

Step - 5

Allocate the next highest range, i.e. Accounts. The requirement of 25 IPs can be fulfilled with 192.168.1.192 /27 (255.255.255.224) IP subnet, which contains 30 valid host IPs. The network number of Accounts department will be 192.168.1.192. The last octet of subnet mask is 11100000.

Step - 6

Allocate next highest range to Management. The Management department contains only 5 computers. The subnet 192.168.1.224 /29 with Mask 255.255.255.248 has exactly 6 valid host IP addresses. So this can be assigned to Management. The last octet of subnet mask will contain 11111000.

By using VLSM, the administrator can subnet the IP subnet such a way that least number of IP addresses are wasted. Even after assigning IPs to every department, the administrator, in this example, still left with plenty of IP addresses which was not possible if he has used CIDR.

IPv4 - Reserved Addresses

There are few Reserved IPv4 address spaces which cannot be used on the internet. These addresses serve special purpose and cannot be routed outside Local Area Network.

Private IP Addresses

Every class of IP, (A, B & C) has some addresses reserved as Private IP addresses. These IPs can be used within a network, campus, company and are private to it. These addresses cannot be routed on Internet so packets containing these private addresses are dropped by the Routers.

In order to communicate with outside world, Internet, these IP addresses must have to be translated to some public IP addresses using NAT process or Web Proxy server can be used.

The sole purpose to create separate range of private addresses is to control assignment of already-limited IPv4 address pool. By using private address range within LAN, the requirement of IPv4 addresses has globally decreased significantly. It has also helped delaying the IPv4 address exhaustion.

IP class, while using private address range, can be chosen as per the size and requirement of the organization. Larger organization may choose class A private IP address range where smaller may opt for class C. These IP addresses can be further sub-netted be assigned to departments within an organization.

Loopback IP Addresses

The IP address range 127.0.0.0 – 127.255.255.255 is reserved for loopback i.e. a Host’s self-address. Also known as localhost address. This loopback IP address is managed entirely by and within the operating system. Using loopback addresses, enable the Server and Client processes on a single system to communicate with each other. When a process creates a packet with destination address as loopback address, the operating system loops it back to itself without having any interference of NIC.

Data sent on loopback is forward by the operating system to a virtual network interface within operating system. This address is mostly used for testing purposes like client-server architecture on a single machine. Other than that, if a host machine can successfully ping 127.0.0.1 or any IP from loopback range, implies that the TCP/IP software stack on the machine is successfully loaded and working.

Link-local Addresses

In case of the Host is not able to acquire an IP address from DHCP server and it has not been assigned any IP address manually, the host can assign itself an IP address from a range of reserved Link-local addresses. Link local address range is 169.254.0.0 - 169.254.255.255.

Assume a network segment where all systems are configured to acquire IP addresses from a DHCP server connected to the same network segment. If the DHCP server is not available, no host on the segment will be able to communicate to any other. Windows (98 or later), and Mac OS (8.0 or later) support this functionality of self-configuration of Link-local IP address. In absence of DHCP server, every host machine randomly chooses an IP address from the above mentioned range and then checks to ascertain by means of ARP, if some other host also has not configured himself with the same IP address. Once all hosts are using link local addresses of same range, they can communicate to each other.

These IP addresses cannot help system to communicate when they do not belong to the same physical or logical segment. These IPs are also not routable.

IPv4 - Example

This section tells how actual communication happens on the Network using Internet Protocol version 4.

Packet flow in network

All the hosts in IPv4 environment are assigned unique logical IP addresses. When a host wants to send some data to another host on the network, it needs the physical (MAC) address of the destination host. To get the MAC address, the host broadcasts ARP message and asks to give the MAC address whoever is the owner of destination IP address. All the host on that segment receives ARP packet but only the host which has its IP matching with the one in ARP message, replies with its MAC address. Once the sender receives the MAC address of receiving station, data is sent on the physical media.

In case, the IP does not belong to the local subnet. The data is sent to the destination by means of Gateway of the subnet. To understand the packet flow we must first understand following components:

MAC Address: Media Access Control Address is 48-bit factory hard coded physical address of network device which can uniquely be identified. This address is assigned by device manufacturers.
Address Resolution Protocol: Address Resolution Protocol is used to acquire the MAC address of a host whose IP address is known. ARP is a Broadcast packet which is received by all the host in the network segment. But only the host whose IP is mentioned in ARP responds to it providing its MAC address.
Proxy Server: To access Internet, network uses Proxy Server which has a public IP assigned. All PCs request Proxy Server for a Server on Internet, Proxy Server on behalf of PC sends the request to server and when it receives response from the Server, the Proxy Server forwards it to the client PC. This is a way to control Internet access in computer networks and it helps to implement web based policies.
Dynamic Host Control Protocol: DHCP is a service by which a host is assigned IP address from a pre-defined address pool. DHCP server also provides necessary information such as Gateway IP, DNS Server Address, lease assigned with the IP etc. By using DHCP services network administrator can manage assignment of IP addresses at ease.
Domain Name System: This is very likely that a user does not know the IP address of a remote Server he wants to connect to. But he knows the name assigned to it for example, tutorialpoints.com. When the user types in the name of remote server he wants to connect to the localhost behind the screens sends a DNS query. Domain Name System is a method to acquire the IP address of the host whose Domain Name is known.
Network Address Translation: Almost all PCs in a computer network are assigned private IP addresses which are not routable on Internet. As soon as a router receives an IP packet with private IP address it drops it. In order to access Servers on public private address, computer networks use an address translation service, which translates between public and private addresses, called Network Address Translation. When a PC sends an IP packet out of a private network, NAT changes the private IP address with public IP address and vice versa.

We can now describe the packet flow. Assume that a user wants to access www.only4programmers.blogspot.com from her personal computer. She is having internet connection from her ISP. The following steps will be taken by the system to help her reach destination website.

Step: 1 – Acquiring an IP Address (DHCP)

When user’s PC boots up, it searches for a DHCP server to acquire an IP address. For the same, PC sends a DHCPDISCOVER broadcast which is received by one or more DHCP servers on the subnet and they all respond with DHCPOFFER which includes all the necessary details like IP, subnet, Gateway, DNS etc. PC sends DHCPREQUEST packet in order to request the offered IP address. Finally, DHCP sends DHCPACK packet to tell PC that it can keep the IP for some given amount of time aka IP lease.

Alternatively a PC can be assigned an IP address manually without taking any help from DHCP Server. When a PC is well configured with IP address details, it can now speak to other computers all over the IP enabled network.

Step: 2 – DNS query

When a user opens a web browser and types www.tutorialpoints.com which is a domain name and a PC does not understand how to communicate with the server using domain names. PC sends a DNS query out on the network in order to obtain the IP address pertaining to the domain name. The pre-configured DNS server responds the query with IP address of the domain name specified.

Step: 3 – ARP request

The PC finds that the destination IP address does not belong to his own IP address range and it has to forward the request to the Gateway. Gateway in this scenario can be a router or a Proxy Server. Though Gateway’s IP address is known to the client machine but computers do not exchange data on IP addresses rather they need machine’s hardware address which is Layer-2 factory coded MAC address. To obtain the MAC address of the Gateway the client PC broadcasts an ARP request saying "Who owns this IP address?" The Gateway in response to the ARP query sends it MAC address. Upon receiving MAC address PC sends the packets to Gateway.

An IP packet has both source and destination addresses and this connects host with a remote host logically. Whereas MAC addresses helps systems on a single network segment to transfer actual data. This is important that source and destination MAC addresses change as they travel across the Internet (segment by segment) but source and destination IP address never changes.

IPv4 - Summary

The Internet Protocol version 4 was designed to be allocated to approx. 4.3 billion addresses. At the beginning of Internet this was considered a much wider address space for which there was nothing to worry about.

The sudden growth in Internet users and its wide spread use has exponentially increase the number of devices which needs real and unique IP to be able to communicate. Gradually, an IP is required by almost every digital equipment which were made to ease human life, such as Mobile Phones, Cars and other electronic devices. The number of devices (other than computers/routers) expanded the demand for extra IP addresses, which were not considered earlier.

Allocation of IPv4 is globally managed by Internet Assigned Numbers Authority (IANA) under coordination with Internet Corporation for Assigned Names and Numbers (ICANN). IANA works closely with Regional Internet Registries, which in turns are responsible for efficiently distribute IP address in their territories. There are five such RIR exist. According to IANA reports, all the IPv4 address blocks have been allocated. To cope up with the situation, as an early steps the following practices were being done:

Private IPs: Few blocks of IPs were declared for private use within a LAN so that the requirement for public IP addresses can be reduced.
NAT: Network address translation is a mechanism by which multiple PCs/hosts with private IP addresses are enabled to access using one or few public IP addresses.
Unused Public IPs were reclaimed by RIRs.

Internet Protocol v6 (IPv6)

IETF (Internet Engineering Task Force) has redesigned IP addresses and to mitigate the drawbacks of IPv4. The new IP address has version 6 and is 128-bit address, by which every single inch of the earth can be given millions of IP addresses.

Today majority of devices running on Internet are using IPv4 and it is not possible to shift them to IPv6 in coming days. There are mechanisms provided by IPv6, by which IPv4 and IPv6 can coexist unless the Internet entirely shifts to IPv6:

Dual IP Stack
Tunneling (6to4 and 4to6)
NAT Protocol Translation.