IPv6 Addressing

Unique Local Unicast IPv6 Address

Unique Local Unicast IPv6 Address

A unique local unicast address is an address that falls within a specific range, but is used only as a global unicast address within an organization. Traffic having a source address within the defined range should not be allowed beyond the borders of the intended domain or onto the public internet.

The current prefix for specifically identifying unique local unicast addresses is fd00/8. The leftmost 64 bits of a unique local unicast address include:

■the well-known prefix “fd”

■a 40-bit global identifier

■a 16-bit subnet identifier

For example:

fd73:110:255:23:215:60ff:fe7a:adc0/64

In the above case, the following values are used with the well-known prefix and L-bit setting:

■global identifier: 0073:110:255

■subnet identifier: 23

■interface identifier: 215:60ff:fe7a:adc0

Unique local unicast addresses can be assigned by router advertisements, DHCPv6 servers, or static configuration. The boundaries for unique local unicast address are set by border routers. Unique local unicast addresses can be assigned in DNS servers supporting an internal network, but should not be included in global DNS assignments.

For related information, refer to:

■RFC 4193: “Unique Local IPv6 Unicast Addresses”

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