Some large blocks of IPv4 addresses, the former Class A network blocks, are assigned in whole to single organizations or related groups of organizations, either by the Internet Corporation for Assigned Names and Numbers (ICANN), through the Internet Assigned Numbers Authority (IANA), or a regional Internet registry. Each block contains 2563 = 224 = 16,777,216 addresses, which covers the whole range of the last three delimited segments of an IP address. This means that 256 /8 address blocks fit into the entire IPv4 space. As IPv4 address exhaustion has advanced to its final stages, some organizations, such as Stanford University, formerly using, have returned their allocated blocks (in this case to APNIC) to assist in the delay of the exhaustion date.

List of reserved /8 blocks

List of assigned /8 blocks to commercial organisations

List of assigned /8 blocks to the United States Department of Defense

List of assigned /8 blocks to the regional Internet registries

The regional Internet registries (RIRs) allocate IPs within a particular region of the world. Note that this list may not include current assignments of /8 blocks to all regional or national Internet registries.

Original list of IPv4 assigned address blocks

The original list of IPv4 address blocks was published in September 1981. In previous versions of the document, network numbers were 8-bit numbers rather than the 32-bit numbers used in IPv4. At that time, three networks were added that were not listed earlier: 42.rrr.rrr.rrr, 43.rrr.rrr.rrr, and 44.rrr.rrr.rrr. The relevant portion of RFC 790 is reproduced here with minor changes: