IPv6 Addressing

Global Unicast Address

Prefixes in Routable IPv6 Addresses

In routable IPv6 addresses, the prefix uniquely identifies an entity and a unicast subnet within that entity, and is defined by a length value specifying the number of leftmost contiguous (high-order) bits comprising the prefix. For an automatically generated global unicast address, the default prefix length is 64 bits. (Practically speaking, the entire prefix in a /64 address defines the subnet.) Prefixes configured through stateful or static methods can be any length compatible with the local network application.

In the following example, the leftmost 64 bits of the address comprise the prefix:

2001:0db8:0000:0212:0215:60ff:fe7a:adc0/64

or

2001:db8::212:215:60ff:fe7a:adc0/64

In this case, the prefix is read as:

2001:0db8:0000:0212::

or

2001:db8::212::

All bits to the right of 0212 comprise the device identifier in the unicast address.

For related information, refer to:

■RFC 3177: “IAB/IESG Recommendations on IPv6 Address Allocations to Sites”

■RFC 4291: “IP Version 6 Addressing Architecture”

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