13-3Enhanced neighbor discovery mechanismThe IPv6 neighbor discovery protocol is implemented through a group of Internet Control MessageProtocol Version 6 (ICMPv6) messages that manage the information exchange between neighbornodes on the same link. The group of ICMPv6 messages takes the place of Address ResolutionProtocol (ARP) messages, Internet Control Message Protocol version 4 (ICMPv4) router discoverymessages, and ICMPv4 redirection messages and provides a series of other functions.Flexible extension headersIPv6 cancels the Options field in the IPv4 header but introduces multiple extension headers to providescalability while improving efficiency. The Options field contains 40 bytes at most, while the size ofIPv6 extension headers is restricted to the maximum size of IPv6 packets.Introduction to IPv6 AddressIPv6 address formatAn IPv6 address is represented as a set of 16-bit hexadecimals, separated by colons. An IPv6 addressis divided into eight groups, and the 16 bits of each group are represented by four hexadecimalnumbers, 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:z Leading zeros in each group can be removed. For example, the above-mentioned address can berepresented in a shorter format as 2001:0:130F:0:0:9C0:876A:130B.z If an IPv6 address contains two or more consecutive groups of zeros, they can be replaced by adouble-colon ::. For example, the above-mentioned address can be represented in the shortestformat as 2001:0:130F::9C0:876A:130B.A double-colon can be used only once in an IPv6 address. Otherwise, the device is unable todetermine how many zeros that double-colons represent when converting them to zeros to restore a128-bit IPv6 address.An IPv6 address consists of two parts: address prefix and interface ID. The address prefix and theinterface 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 the IPv6-address is inany of the notations above mentioned, and prefix-length is a decimal number indicating how many bitsfrom the left-most of an IPv6 address is the address prefix.IPv6 address classificationIPv6 addresses fall into three types: unicast address, multicast address, and anycast address.z Unicast address: An identifier for a single interface, similar to an IPv4 unicast address. A packetsent to a unicast address is delivered to the interface identified by that address.
