Information Technology Reference
In-Depth Information
address's 3 high-order bits. These classes are relevant only when classful routing is
performed. Multicast addresses occur when the address's 3 high-order bits are all 1s.
The IPv6 addressing format includes an increased level of aggregation, which is reflected
in the addresses themselves using Aggregator Level fields. IPv6 has unicast, multicast, and
anycast addresses. The broadcast address type has been removed. The additional bits in an
IPv6 address allow much more information about the actual use and assigning authorities
to be reflected in the address itself.
Anycast Address Functionality
The concept of an anycast address is formalized in IPv6. An anycast address has no special
notation or format; it is a unicast address that has been assigned to multiple interfaces or
devices. Data sent to any anycast address is delivered to the closest instance of that address
as determined by the routing protocol. Informally, anycast functionality is available in IPv4,
as discussed in Chapter 11, “Multiprotocol BGP and Interdomain Multicast.”
General Address Format
In IPv4, addresses are 32 bits or 4 bytes in length and are typically represented in a dotted-
quad format. In IPv6, addresses are 128 bits or 16 bytes and are represented in a colon-
delimited fashion, with eight sequences of hexadecimal digits in 2-byte increments, as
follows:
2001:0400:AAAA:BBBB:CCCC:DDDD:1234:5678
Working with IPv6 addresses is more cumbersome because of their length. It is much more
difficult to differentiate between addresses at a glance. To make it easier to work with IPv6
addresses, you can abbreviate them when they contain two or more consecutive 0s. Here's
a sample address with consecutive 0s:
2001:0400:0000:0000:0000:0000:1234:5678
The abbreviated form of this address is
2001:0400::1234:5678
You can abbreviate only a single sequence of consecutive 0s in an IPv6 address. If you tried
to abbreviate multiple sequences of consecutive 0s, you would have no way to determine
how many 0s should be inserted when expanding the address to its full length.
Another form of abbreviation is to leave out the leading 0s in each octet. An IPv6 address
with leading 0s might look like this:
2001:0400:00AA:00BB:00CC:00DD:1234:5678
The abbreviated form of the address looks like this:
2001:400:AA:BB:CC:DD:1234:5678
