IPv6

IPv6

Internet Protocol version 6 (IPv6) will succeed IPv4 as the standard networking protocol of the Internet. IPv6 provides a number of advances over IPv4 but the primary reason for its replacing IPv4 is its limitation in addresses. IPv4 uses 32 bit addresses which means there is a theoretical limit of 2 to the power of 32. The IPv6 address scheme is based on a 128 bit address or a theoretical limit of 2 to the power of 128.

IPv6 addressing

Possible addresses:

l IPv4 = 4,294,967,296 (over 4 billion) l IPv6 = 340,282,366,920,938,463,463,374,607,431,768,211,456 (over 340 undecillion – We had to look that term up. We didn’t know what a number followed by 36 digits was either)

Assuming a world population of approximately 8 billion people, IPv6 would allow for each individual to have approximately 42,535,295,865,117,200,000,000,000,000 devices with an IP address. That’s 42 quintillion devices.

There is little likelihood that you will ever need to worry about these numbers as any kind of serious limitation in addressing but they do give an idea of the scope of the difference in the available addressing.

IPv6 address syntax

Aside from the difference of possible addresses there is also the different formatting of the addresses that will need to be addressed.

A computer would view an IPv4 address as a 32 bit string of binary digits made up of 1s and 0s, broken up into 4 octets of 8 digits separated by a period “.” Example:

10101100.00010000.11111110.00000001

To make number more user friendly for humans we translate this into decimal, again 4 octets separated by a period “.” which works out to:

172.16.254.1

A computer would view an IPv6 address as a 128 bit string of binary digits made up of 1s and 0s, broken up into 8 octets of 16 digits separated by a colon “:”

1000000000000001:0000110110111000:101011000001000:1111111000000001:000000000000000

0:0000000000000000:0000000000000000:0000000000000000

To make number a little more user friendly for humans we translate this into hexadecimal, again 8 octets separated by a colon “:” which works out to:

8001:0DB8:AC10:FE01:0000:0000:0000:0000:

IPv6 packet structure

Because any four-digit group of zeros within an IPv6 address may be reduced to a single zero or altogether omitted, this address can be shortened further to:

8001:0DB8:AC10:FE01:0:0:0:0 or

8001:0DB8:AC10:FE01::

IPv6 packet structure

Each IPv6 packet consists of a mandatory fixed header and optional extension headers, and carries a payload, which is typically either a datagram and/or Transport Layer information. The payload could also contain data for the Internet Layer or Link Layer. Unlike IPv4, IPv6 packets aren’t fragmented by routers, requiring hosts to implement Maximum Transmission Unit (MTU) Path Discovery for MTUs larger than the smallest MTU (which is 1280 octets).

Jumbograms and jumbo payloads

In IPv6, packets which exceed the MTU of the underlying network are labeled jumbograms, which consist of a jumbo payload. A jumbogram typically exceeds the IP MTU size limit of 65,535 octets, and provides the jumbo payload option, which can allow up to nearly 4GiB of payload data, as defined in RFC 2675. When the MTU is determined to be too large, the receiving host sends a ‘Packet too Big’ ICMPv6 type 2 message to the sender.

Fragmentation and reassembly

As noted, packets that are too large for the MTU require hosts to perform MTU Path Discovery to determine the maximum size of packets to send. Packets that are too large require a ‘Fragment’ extension header, to divide the payload into segments that are 8 octets in length (except for the last fragment, which is smaller). Packets are reassembled according to the extension header and the fragment offset.

Benefits of IPv6

In addition to the expanded number of addresses, some of the other benefits of IPv6 include:

l More efficient routing l Reduced management requirement l Stateless auto-reconfiguration of hosts l Improved methods to change Internet Service Providers l Better mobility support l Multi-homing l Security l Scoped address: link-local, site-local and global address space Benefits of IPv6


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