Figure 261 Provider Based Unicast Address Format RFC 1884

Registry ID {5 bits)

Provider ID (A/bits)

Subscriber ID (56 - A/bits)

Intra-Subscriber ID (64 bits)

The following list describes Figure 26-1, defining the unicast address format in detail.

• Format prefix— Indicates the type of address as provider-based unicast. This is always 3 bits, and it is coded as 010.

• Registry ID— Identifies the Internet address registry from which this ISP obtains addresses. This is a 5-bit value indicating the Internet Assigned Numbers Authority (IANA) or one of the three regional registries, namely the Internet Network Information Center (InterNIC), Réseaux IP Européens Network Coordination Center (RIPE NCC), or Asia-Pacific Network Information Center (APNIC). In the future, national registries might also be accommodated.

• Provider ID— Identifies the ISP. This field contains the address block that the address registry authority assigns to the ISP.

• Subscriber ID— Identifies the ISP's subscriber. This field contains the address that the ISP assigns to the subscriber. The ProviderID and SubscriberID fields together are 56 bits in length.

• Intra-Subscriber ID— Contains the portion of the address assigned and managed by the subscriber. The 64bit value used comprises a 16-bit subnet identifier and a 48-bit interface identifier (such as an IEEE MAC address).

With more than 16 million hosts using 32-bit addresses, the public Internet must continue to accommodate IPv4 addresses as it migrates to IPv6 and IPv6 addressing. IPv4 addresses are carried in a 128-bit IPv6 address that begins with 80 zeros (0:0:0:0:0).

The next 16-bit block contains the compatibility bits, which indicate the way in which the host/router handles IPv4 and IPv6 addresses. If the device can handle either IPv4 or IPv6 addresses, the compatibility bits are all set to zero

(0) and termed an IPv4-compatible IPv6 address. If the address represents an IPv4-only node, the compatibility bits are all set to one (OxFFFF), and the address is termed an IPv4-mapped IPv6 address. The final 32 bits contain a 32bit IPv4 address in dotted decimal form.

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