3Com Switch 8800 Advanced Software V5 Configuration Guide

IPv6 Overview 213
Enhanced neighbor discovery mechanism
The IPv6 neighbor discovery protocol is a group of Internet control message
protocol version 6 (ICMPv6) messages that manages the information exchange
between neighbor nodes on the same link. The group of ICMPv6 messages takes
the place of address resolution protocol (ARP) message, Internet control message
protocol version 4 (ICMPv4) router discovery message, and ICMPv4 redirection
message to provide a series of other functions.
Flexible extension headers
IPv6 cancels the Options field in IPv4 packets but introduces multiple extension
headers. In this way, IPv6 enhances the flexibility greatly to provide scalability for IP
while improving the handling efficiency. The Options field in IPv4 packets contains
40 bytes at most, while the size of IPv6 extension headers is restricted by that of
IPv6 packets.
Introduction to IPv6
Address
IPv6 address format
An IPv6 address is represented as a series of 16-bit hexadecimals, separated by
colons. An IPv6 address is divided into eight groups, and the 16 bits of each group
are represented by four hexadecimal numbers which are separated by colons, for
example, 2001:0000:130F:0000:0000:09C0:876A:130B.
To simplify the representation of IPv6 addresses, zeros in IPv6 addresses can be
handled as follows:
■ Leading zeros in each group can be removed. For example, the
above-mentioned address can be represented in shorter format as
2001:0:130F:0:0:9C0:876A:130B.
■ If an IPv6 address contains two or more consecutive groups of zeros, they can
be replaced by the double-colon :: option. For example, the above-mentioned
address can be represented in the shortest format as
2001:0:130F::9C0:876A:130B.
c
CAUTION: The double-colon :: option can be used only once in an IPv6 address.
Otherwise, the device is unable to determine how many zeros double-colons
represent when converting them to zeros to restore a 128-bit IPv6 address.
An IPv6 address consists of two parts: address prefix and interface ID. The address
prefix and the interface ID are respectively equivalent to the network ID and the
host ID in an IPv4 address.
An IPv6 address prefix is written in IPv6-address/prefix-length notation, where
IPv6-address is an IPv6 address in any of the notations and prefix-length is a
decimal number indicating how many bits from the utmost left of an IPv6 address
are the address prefix.
IPv6 address classification
IPv6 addresses fall into three types: unicast address, multicast address, and anycast
address.
■ Unicast address: An identifier for a single interface, similar to an IPv4 unicast
address. A packet sent to a unicast address is delivered to the interface
identified by that address.