Administrator Guide

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.
IPv6 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.
Link-local Addresses
Link-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 instant
automatic network connectivity for any IPv6 host and means that if several hosts connect to a common hub or switch, they have an
instant communication path via their link-local IPv6 address.
Link-local addresses cannot be routed to the public Internet.
Static and Dynamic Addressing
Static 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 IPv6
addresses assigned using DHCP may stay the same for long periods of time, they can change. In some cases, a network administrator may
implement dynamically assigned static IPv6 addresses. In this case, a DHCP server is used, but it is specifically configured to always assign
the same IPv6 address to a particular computer, and never to assign that IP address to another computer. This allows static IPv6
addresses to be configured in one place, without having to specifically configure each computer on the network in a different way.
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IPv6 Routing