1-3Enhanced neighbor discovery mechanismThe IPv6 neighbor discovery protocol is implemented by a group of Internet Control Message ProtocolVersion 6 (ICMPv6) messages. The IPv6 neighbor discovery protocol manages message exchangebetween neighbor nodes (nodes on the same link). The group of ICMPv6 messages takes the place ofAddress Resolution Protocol (ARP), Internet Control Message Protocol Version 4 (ICMPv4), andICMPv4 redirect messages to provide a series of other functions.Flexible extension headersIPv6 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 processing efficiency.The Options field in IPv4 packets contains only 40 bytes, while the size of IPv6 extension headers isrestricted by that of IPv6 packets.Introduction to IPv6 AddressIPv6 addressesAn IPv6 address is represented as a series of 16-bit hexadecimals, separated by colons. An IPv6address is divided into eight groups, 16 bits of each group are represented by four hexadecimalnumbers 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:z Leading zeros in each group can be removed. For example, the above-mentioned address can berepresented in 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 thedouble-colon :: option. For example, the above-mentioned address can be represented in theshortest format as 2001:0:130F::9C0:876A:130B.The double-colon :: can be used only once in an IPv6 address. Otherwise, the device is unable todetermine how many zeros the double-colon represents when converting it to zeros to restore the IPv6address to a 128-bit 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 IPv6-address is an IPv6address in any of the notations and prefix-length is a decimal number indicating how many bits from theleft of an IPv6 address are the address prefix.IPv6 address classificationIPv6 addresses mainly 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.
