Mark Mentovai

IPv4 Address Space Allocations

IANA (Internet Assigned Numbers Authority), the top-level administrator of Internet address space, currently allocates IPv4 space to RIRs (Regional Internet Registries) for delegation to end-users. These RIRs are ARIN (American Registry for Internet Numbers), RIPE (Réseau IP Européens), APNIC (Asia-Pacific Network Information Center), and LACNIC (Latin American and Caribbean Internet Address Registry). Each RIR delegates address space in accordance with its own policies. These policies vary between RIRs, and are subject to change.

In the interests of implementing prefix filtering that accurately reflects the state of the Internet, it is useful to determine the minimum block size that has been delegated to end organizations over various segments of the entire address space. This information can be used as a starting point for devising a fair prefix filtering policy.

¹ The 24.0.0.0/8 network has been designated for data-over-cable networks; it is administered at the top level by ARIN but allocations may be made to regions outside of the Americas.
² The 200.0.0.0/8 network was initially administered by ARIN for the Latin American-Caribbean region prior to its reassignment to LACNIC in 2002. Under the administration of ARIN, allocations in blocks as small as /24 were made.
³ While LACNIC does not currently publish minimum allocation sizes in the same manner as other RIRs, it only allocates address space in blocks of /20 and larger, per LACNIC IPv4 Address Block Allocation Policies.
Data compiled from IANA's IPv4 Address Space Assignments and information supplied by RIRs as of 2004 July 5. For more information, see minimum allocation information for ARIN, RIPE, APNIC, and LACNIC.

Some parts of the global IPv4 address space have been reserved for special purposes. These blocks have been designated for such uses as private internets and multicast traffic.

¹ IANA maintains the list of X.25/X.121 public data network numbers.
This is not an exhaustive list of reserved space. See Special-Use IPv4 Addresses (RFC 3330).

A good deal of IPv4 space has not yet been assigned, or (in the case of some former class-A space) was assigned and then returned to IANA. It is now reserved space. This table summarizes all such blocks.

¹ 96.0.0.0/3 is unassigned with the exception of 127.0.0.0/8, which is reserved as the loopback network. See above.
Data collected from IANA's IPv4 Address Space Assignments, current as of 2004 July 5.

For address space not listed above, allocations made prior to CIDR (Classless Inter-Domain Routing) follow the classful guidelines below.

Note that not all allocations made in this space conform to classful allocation prefix lengths.

For additional information, see:

• Internet Protocol (RFC 791, STD 5)
• An Architecture for IP Address Allocation with CIDR (RFC 1518)
• Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy (RFC 1519)
• Address Allocation for Private Internets (RFC 1918, BCP 5)
• Internet Registry IP Allocation Guidelines (RFC 2050, BCP 12)
• Special-Use IPv4 Addresses (RFC 3330)
• IPv4 Address Space Assignments (IANA)
• IANA (Internet Assigned Numbers Authority)
• ARIN (American Registry for Internet Numbers) and ARIN allocation statistics
• RIPE (Réseau IP Européens) and RIPE allocation statistics
• APNIC (Asia-Pacific Network Information Center) and APNIC allocation statistics
• LACNIC (Latin American and Caribbean Internet Address Registry) and LACNIC IPv4 Address Block Allocation Policies

2000 November 19
2004 July 5
Mark Mentovai / contact