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Internet Protocol Version 6 also known as IPv6 or IPng (Internet Protocol Next Generation) is a low level protocol designed to replace the IPv4 protocol defined in RFC 791 that has been the backbone of the Internet since the mid 1970’s. The basic reason for replacing IPv4 with IPv6 is the imminent exhaustion of the IPv4 32 bit address space. The most significant and best known change is the expansion of the address space from 32 bit to 128 bits but the format of IP packets has also been changed and the associated ICMP formats have also been revised. IPv6 is described in RFC 1883.

IPv6 Addresses

The preferred way of writing an IPv6 address is a group of eight 16-bit pieces represented in hexadecimal, Leading zeroes may be suppressed on the individual pieces giving addresses such as

FEDC:AB19:12FE:0234:98EF:1178:8891:CAFF 1090:0:0:0:A:200:20:CAFF

It is also permissible to concatenate a sequence of zero values as “::”, so the second example above could be written as

1090::A:200:20:CAFF An IPv6 address specifically identifies an interface rather than a host or node as with IPv4. This is not as revolutionary as it might appear since multi-homed hosts are common in IPv4 networks, with each port being in a different sub-net.

ICMPv6

ICMPv6 is related to IPv6 in the same way as ICMPv4 (RFC792) is related to IPv4. The first 10 words of an ICMPv6 header are the same as the first 10 words of an IPv6 header with the next header field set to 58. This is followed by a single word encoding the ICMP type and code in a single byte each followed by a 2 byte checksum. ICMPv6 is described in RFC 1885.

Source: http://www.scit.wlv.ac.uk/~jphb/comms/ipv6.html