Administrator Guide

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).
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.
● 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 one or more options. The first byte if the field identifies the Option type, and directs the router how to
handle the option.
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.
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 and/or trailing
zeros in a group can 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::1428:57ab
IPv6 networks are written using classless inter-domain routing (CIDR) notation. An IPv6 network (or subnet) is a contiguous
group of IPv6 addresses 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 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 through
2001:0db8:1234:ffff:ffff:ffff:ffff:ffff.
IPv6 Routing
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