The upper 64 bits of an IPv6 address are composed from a global routing prefix plus a subnet ID, as defined in RFC 3513. A general prefix (for example, /48) holds a short prefix, based on which a number of longer, more-specific prefixes (for example, /64) can be defined. When the general prefix is changed, all of the more-specific prefixes based on it will change, too. This function greatly simplifies network renumbering and allows for automated prefix definition.
For example, a general prefix might be 48 bits long ("/48") and the more specific prefixes generated from it might be 64 bits long ("/64"). In the following example, the leftmost 48 bits of all the specific prefixes will be the same, and they are the same as the general prefix itself. The next 16 bits are all different.
General prefix: 2001:DB8:2222::/48Specific prefix: 2001:DB8:2222:0000::/64Specific prefix: 2001:DB8:2222:0001::/64Specific prefix: 2001:DB8:2222:4321::/64Specific prefix: 2001:DB8:2222:7744::/64
General prefixes can be defined in several ways:
- Based on a 6to4 interface
- Dynamically, from a prefix received by a Dynamic Host Configuration Protocol (DHCP) for IPv6 prefix delegation client
More specific prefixes, based on a general prefix, can be used when configuring IPv6 on an interface.