488 | IPv6 Addressing
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Hop-by-Hop Options header
The Hop-by-Hop options header contains information that is examined by every router along the packet’s
path. 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 any
router 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 not
need 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 same
values 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 include
the 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, and
directs 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 1
if it can change; the value is 0 if it cannot change.
Addressing
IPv6 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 may be omitted and replaced with two colons(::). For
example, 2001:0db8:0000:0000:0000:0000:1428:57ab can be shortened to 2001:0db8::1428:57ab. Only
one set of double colons is supported in a single address. Any number of consecutive 0000 groups may be
reduced to two colons, as long as there is
only one double colon used in an address. Leading zeros in a group
can also be omitted (as in ::1 for localhost).
All the addresses in the following list are all valid and equivalent.
00 Skip and continue processing
01 Discard the packet.
10 Discard the packet and send an ICMP Parameter Problem Code 2 message to the
packet’s Source IP Address identifying the unknown option type
11 Discard the packet and send an ICMP Parameter Problem, Code 2 message to the
packet’s Source IP Address only if the Destination IP Address is not a multicast address.
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