10 Discard the packet and send an ICMP Parameter Problem Code 2 message to the packet’s Source IPAddress identifying the unknown option type.11 Discard the packet and send an ICMP Parameter Problem, Code 2 message to the packet’s Source IPAddress only if 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 0if it cannot 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, thezeros may be omitted and replaced with two colons(::). For example, 2001:0db8:0000:0000:0000:0000:1428:57ab can be shortenedto 2001:0db8::1428:57ab. Only one set of double colons is supported in a single address. Any number of consecutive 0000 groupsmay be reduced to two colons, as long as there is only one double colon used in an address. Leading and/or trailing zeros in a groupcan also be omitted (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 groupof IPv6 addresses the size of which must be a power of two; the initial bits of addresses, which are identical for all hosts in thenetwork, are called the 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 a single 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 providesinstant automatic network connectivity for any IPv6 host and means that if several hosts connect to a common hub or switch, theyhave an instant 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). Eventhough IPv6 addresses assigned using DHCP may stay the same for long periods of time, they can change. In some cases, a networkadministrator may implement dynamically assigned static IPv6 addresses. In this case, a DHCP server is used, but it is specificallyconfigured to always assign the same IPv6 address to a particular computer, and never to assign that IP address to anothercomputer. This allows static IPv6 addresses to be configured in one place, without having to specifically configure each computer onthe network in a different way.422 IPv6 Routing
