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3-18
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. (Pratically 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”