Information Technology Reference
In-Depth Information
The value 0xFE80 in the first 10 bits of the link-local address is the global prefix that is
allocated for link-local addressing. All link-local addresses begin with this prefix.
Site-local addressing was designed for use within a site. A site may consist of only a single
link, or it might have thousands of links and devices. Site-local addressing is not globally
routable, and there is no guarantee that a host with a site-local address is unique outside
the scope of the site it resides in. Site-local addressing is the IPv6 equivalent of private
addressing in IPv4. IPv6 traffic with a site-local source or destination address must not be
forwarded outside the site. The format of a site-local address is shown in Figure 12-3.
Figure 12-3
Site-Local Addressing Format
10 Bits
38 Bits
16 Bits
64 Bits
1111 1110 11 (0xFEC0)
0
Subnet ID
Interface Identifier
The value 0xFEC0 in the first 10 bits of the site-local address is the global prefix that is
allocated for site-local addressing. All site-local addresses begin with this prefix.
Interface Identifiers
Interface Identifiers are a component of unicast IPv6 addresses. The Interface ID is used to
identify an interface on a link. The Interface ID must be unique over the subnet. Although
it isn't required, it is recommended that the Interface ID be unique over the link. The case
where an Interface ID may not be unique over a link is when multiple subnets are assigned
to a single link.
All unicast addresses that do not begin with a binary format prefix of 000 must have a
64-bit Interface ID, which is constructed in Modified EUI-64 format. The construction
of Modified EUI-64-based Interface ID is described in RFC 3513. When possible, the
Interface ID is based on the 48-bit MAC address.
Special Addresses
Two special addresses have been defined for IPv6: the
unspecified address
and the
loopback
address
. The unspecified address is 0:0:0:0:0:0:0:0, or :: in abbreviated form. Never assign
the unspecified address to a node, because it indicates the absence of an address. A node
may source packets with the unspecified address before it has fully initialized and obtained
its own address. This is done as part of an address configuration process. Packets with a
source or destination address set to the unspecified address should not be forwarded by an
IPv6 router. The unspecified address should never be used as a destination address.
