IPv4 explained

What is IPv4?

IPv4 (Internet Protocol version 4, RFC 791, 1981) is the addressing protocol that has carried most Internet traffic for decades. Every address is a 32-bit number, written as four decimal octets such as 192.0.2.1, giving 4 294 967 296 (2³²) possible addresses.

Its purpose is to identify hosts and networks and let routers forward packets between them. The entire space is the block 0.0.0.0/0. It is managed as a hierarchy by IANA and the five RIRs, with a number of blocks set aside by the IETF for special technical roles.

Special-purpose ranges

These blocks are reserved and are not handed to RIRs for ordinary assignment. They are listed in the IANA IPv4 Special-Purpose registry (framework: RFC 6890):

The In BGP column shows whether a block may legitimately appear in the global routing table. Only global unicast space (the RIR-delegated blocks) is meant to be announced in BGP; every special-purpose block below is purely technical and must never be routed on the public Internet — if one shows up in BGP it is a bogon (a leak or hijack). The one historical exception is 192.88.99.0/24, the 6to4 anycast prefix, now deprecated.

Allocation to the RIRs

The remaining unicast space (roughly 1.0.0.0/8 – 223.255.255.255) was distributed by IANA to the five Regional Internet Registries in /8 blocks. On 3 February 2011 IANA allocated its last five /8s and the central free pool was exhausted — RIRs now manage only recovered and returned space. The full per-block map is the IANA IPv4 Address Space registry; live per-RIR holdings are in the RIR delegated statistics.

See the live IPv4 picture on our IPv4 address-space map and BGP Full View.

Every IPv4 /8 block

The whole IPv4 space is exactly 256 /8 blocks. Below is the complete, sequential map from the IANA IPv4 Address Space registry — which blocks are routable (delegated to a RIR: APNIC RIPE NCC ARIN LACNIC AFRINIC; early blocks are ARIN-administered legacy) and which are technical / reserved (private, loopback, multicast, future use). Scroll the table.