488 | IPv6 Addressingw w w . d e l l . c o m | s u p p o r t . d e l l . c o m Hop-by-Hop Options headerThe Hop-by-Hop options header contains information that is examined by every router along the packet’spath. It follows the IPv6 header and is designated by the Next Header value 0 (zero) (Table 22-1).When a Hop-by-Hop Options header is not included, the router knows that it does not have to process anyrouter specific information and immediately processes the packet to its final destination.When a Hop-by-Hop Options header is present, the router only needs this extension header and does notneed to take the time to view further into the packet.The Hop-by-Hop Options header contains:• Next Header (1 byte)This field identifies the type of header following the Hop-by-Hop Options header and uses the samevalues shown in Table 22-1.• Header Extension Length (1 byte)This field identifies the length of the Hop-by-Hop Options header in 8-byte units, but does not includethe first 8 bytes. Consequently, if the header is less than 8 bytes, the value is 0 (zero).• Options (size varies)This field can contain 1 or more options. The first byte if the field identifies the Option type, anddirects the router how to handle the option.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 1if it can change; the value is 0 if it cannot change.AddressingIPv6 addresses are normally written as eight groups of four hexadecimal digits, where each group isseparated 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 may be omitted and replaced with two colons(::). Forexample, 2001:0db8:0000:0000:0000:0000:1428:57ab can be shortened to 2001:0db8::1428:57ab. Onlyone 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 zeros in a groupcan also be omitted (as in ::1 for localhost).All the addresses in the following list are all valid and equivalent.00 Skip and continue processing01 Discard the packet.10 Discard the packet and send an ICMP Parameter Problem Code 2 message to thepacket’s Source IP Address identifying the unknown option type11 Discard the packet and send an ICMP Parameter Problem, Code 2 message to thepacket’s Source IP Address only if the Destination IP Address is not a multicast address.
