Information Technology Reference
In-Depth Information
You can abbreviate leading 0s only. You can't abbreviate trailing 0s. This is because if both
leading and trailing 0s could be abbreviated, IPv6 devices would not know where to add
back the 0s when expanding an abbreviated address to its full length.
There's another way to abbreviate an IPv6 address that contains a significant number of 0s.
With multiple strings of consecutive 0s, one is compressed to a double colon, and the other
has leading 0s compressed.
Here is an IPv6 address with multiple strings of consecutive 0s and leading 0s:
2001:0400:0000:0000:ABCD:0000:0000:1234
Here's the abbreviated address:
2001:400:0:0:ABCD::1234
Figure 12-1 shows the general format of an IPv6 address.
Figure 12-1
Format of an IPv6 Address
n bits
m bits
128 - m - n bits
Global Routing Prefix
Subnet ID
Interface Identifier
The actual type of IPv6 address defines the values for m and n in Figure 12-1. These fields
are not a fixed length, but the total number of bits for each address is 128 bits. The type of
address is defined by the prefix itself.
IPv6 has several types of addressing, such as link-local, site-local, globally aggregatable,
and addressing formats for IPv4 and IPv6 compatibility. In IPv4, the entire address space
is divided into Class A, B, C, D, and E space. An address's class is indicated in the first 4
bits of the address. The use of class membership for an address for the purpose of autosum-
marization is deprecated with classless routing and Classless Interdomain Routing (CIDR).
Table 12-1 shows the original address classes for IPv4.
Table 12-1
IPv4 Address Space
Address Class
High-Order Byte
Address Space
Class A
0
XXXXXXX
0.0.0.0/1
Class B
10
XXXXXX
128.0.0.0/2
Class C
110
XXXXX
192.0.0.0/3
Class D
1110
XXXX
224.0.0.0/4
Class E
1111
XXXX
248.0.0.0/4
