Working with 128-bit numbers is difficult, so the IPv6 address notation represents the 128 bits as 32 hexadecimal values. The 32 hexadecimal values are further subdivided into eight fields of four hexadecimal values, using colons as delimiters. Each field of four hexadecimal values is called a block.
The IPv6 address has a three-part hierarchy, as shown in Figure 1. The global prefix, also called a site prefix, is the first three blocks of the address and is assigned to an organization by an Internet names registry. The subnet ID includes the fourth block of the address, and the interface ID includes the last four blocks of the address. The network administrator controls both the subnet and interface ID.
As an example, if a host has an IPv6 address 3ffe:6a88:85a3:08d3:1319:8a2e:0370:7344, the global prefix address is fe80:6a88:85a3, the subnet ID address is 08d3, and the interface ID address is 1319:8a2e:0370:7344.
An IPv6 address can be abbreviated with the following rules:
- Omit leading zeroes in a 16-bit value.
- Replace one group of consecutive zeroes by a double colon.
Figure 2 is an example of how these rules are applied.