Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 426 — #452
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double colon. However, only one such :: is allowed per address. This kind
of shorthand notation is shown in Example 21.3, where all three lines repre-
sent the same address.
Example 21.3: Sample IPv6 Address
fe80 : 0000 : 0000 : 0000 : 0000 : 10 : 1000 : 1a4
fe80 : 0 : 0 : 0 : 0 : 10 : 1000 : 1a4
fe80 : : 10 : 1000 : 1a4
Each part of an IPv6 address has a defined function. The first bytes form
the prefix and specify the type of address. The center part is the network
portion of the address, but it may be unused. The end of the address forms
the host part. With IPv6, the netmask is defined by indicating the length of
the prefix after a slash at the end of the address. An address, as shown in
Example 21.4, contains the information that the first 64 bits form the net-
work part of the address and the last 64 form its host part. In other words,
the 64 means that the netmask is filled with 64 1-bit values from the left.
Just like with IPv4, the IP address is combined with AND with the values
from the netmask to determine whether the host is located in the same sub-
network or in another one.
Example 21.4: IPv6 Address Specifying the Prefix Length
fe80::10:1000:1a4/64
IPv6 knows about several predefined types of prefixes, some of which are
shown in Table 21.4.
Table 21.4: Various IPv6 Prefixes
Prefix (hex) Definition
00 IPv4 addresses and IPv4 over IPv6 compatibility
addresses. These are used to maintain compatibility
with IPv4. Their use still requires a router able to
translate IPv6 packets into IPv4 packets. Several
special addresses, (such as the one for the loopback
device, have this prefix as well.
426 21.2. IPv6 — The Next Generation Internet