Global Aggregatable Unicast Addresses

KEY POINT

IPv6 global aggregatable unicast addresses are equivalent to IPv4 unicast addresses.

The structure of global aggregatable unicast addresses enables summarization (aggregation) of routing prefixes so that the number of routing table entries in the global routing table can be reduced. Global unicast addresses used on links are aggregated upward, through organizations, and then to intermediate-level ISPs, and eventually to top-level ISPs. A global unicast address typically consists of a 48-bit global routing prefix, a 16-bit subnet ID, and a 64-bit interface ID (typically in EUI-64 bit format), as illustrated in Figure 6-19.

Figure 6-19 IPv6 Global Aggregatable Unicast Address Structure

Network Subnet Host

48 Bits 16 Bits 64 Bits

Interface ID

3 Bits

The subnet ID can be used by individual organizations to create their own local addressing hierarchy using subnets. This field allows an organization to use up to 65,536 individual subnets.

A fixed prefix of binary 2000::/3 (binary 001) indicates a global aggregatable IPv6 address; this is the current range of IPv6 global unicast addresses assigned by the Internet Assigned Numbers Authority (IANA). Assignments from this block are registered in the IANA registry, which is available at http://www.iana.org/assignments/ipv6-unicast-address-assignments.

The 64-bit Interface ID field identifies interfaces on a link and therefore must be unique on the link.

NOTE RFC 3587, IPv6 Global Unicast Address Format, defines the global unicast address format.

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