10 Discard the packet and send an ICMP Parameter Problem Code 2 message to the packet’s Source IP Addressidentifying the unknown option type.11 Discard the packet and send an ICMP Parameter Problem, Code 2 message to the packet’s Source IP Address onlyif the Destination IP Address is not a multicast address.The second byte contains the Option Data Length.The third byte specifies whether the information can change en route to the destination. The value is 1 if it can change; the value is 0 if itcannot change.AddressingIPv6 addresses are normally written as eight groups of four hexadecimal digits, where each group is separated by a colon (:).For example, 2001:0db8:0000:0000:0000:0000:1428:57ab is a valid IPv6 address. If one or more four-digit group(s) is 0000, the zeros maybe omitted and replaced with two colons(::). For example, 2001:0db8:0000:0000:0000:0000:1428:57ab can be shortened to2001:0db8::1428:57ab. Only one set of double colons is supported in a single address. Any number of consecutive 0000 groups may bereduced to two colons, as long as there is only one double colon used in an address. Leading and/or trailing zeros in a group can also beomitted (as in ::1 for localhost, 1:: for network addresses and :: for unspecified addresses).All the addresses in the following list are all valid and equivalent.• 2001:0db8:0000:0000:0000:0000:1428:57ab• 2001:0db8:0000:0000:0000::1428:57ab• 2001:0db8:0:0:0:0:1428:57ab• 2001:0db8:0:0::1428:57ab• 2001:0db8::1428:57ab• 2001:db8::1428:57abIPv6 networks are written using classless inter-domain routing (CIDR) notation. An IPv6 network (or subnet) is a contiguous group of IPv6addresses the size of which must be a power of two; the initial bits of addresses, which are identical for all hosts in the network, are calledthe network's prefix.A network is denoted by the first address in the network and the size in bits of the prefix (in decimal), separated with a slash. Because asingle host is seen as a network with a 128-bit prefix, host addresses may be written with a following /128.For example, 2001:0db8:1234::/48 stands for the network with addresses 2001:0db8:1234:0000:0000:0000:0000:0000 through2001:0db8:1234:ffff:ffff:ffff:ffff:ffff.Link-local AddressesLink-local addresses, starting with fe80:, are assigned only in the local link area.The addresses are generated usually automatically by the operating system's IP layer for each network interface. This provides instantautomatic network connectivity for any IPv6 host and means that if several hosts connect to a common hub or switch, they have aninstant communication path via their link-local IPv6 address.Link-local addresses cannot be routed to the public Internet.Static and Dynamic AddressingStatic IPv6 addresses are manually assigned to a computer by an administrator.Dynamic IPv6 addresses are assigned either randomly or by a server using dynamic host configuration protocol (DHCP). Even though IPv6addresses assigned using DHCP may stay the same for long periods of time, they can change. In some cases, a network administrator mayimplement dynamically assigned static IPv6 addresses. In this case, a DHCP server is used, but it is specifically configured to always assignIPv6 Routing 455
