AS numbers are assigned in blocks by Internet Assigned Numbers Authority (IANA) to regional Internet registries (RIRs). The appropriate RIR then assigns ASNs to entities within its designated area from the block assigned by IANA. Entities wishing to receive an ASN must complete the application process of their RIR, LIR or upstream service provider[15][16] and be approved before being assigned an ASN. Current IANA ASN assignments to RIRs can be found on the IANA website.[17] RIRs, as part of NRO, can revoke AS numbers as part of their Internet governance abilities.[18]
There are other sources for more specific data:
AS-SET[edit]
Autonomous systems can be included in one or more AS-SETs, for example AS-SET of RIPE NCC "AS-12655" has AS1, AS2 and AS3 as its members,[22] but AS1 is also included in other sets in ARIN (AS-INCAPSULA) and APNIC (AS-IMCL). Another AS-SET sources can be RADB, LEVEL3 (tier 1 network now called Lumen Technologies) and also ARIN has ARIN-NONAUTH source of AS-SETs.[23] AS-SETs are created by network operators in an Internet Routing Registry (IRR), like other route objects, and can be included in other AS-SETs and even form cycles.[24][25]
AS-SET names usually start with "AS-", but can also have a hierarchical name. For example, the administrator of AS 64500 may create an AS-SET called "AS64500:AS-UPSTREAMS", to avoid conflict with other similarly named AS-SETs.[26]
AS-SETs are often used to simplify management of published routing policies. A routing policy is published in the IRR using "import" and "export" (or the newer "mp-import" and "mp-export") attributes, which each contain the source or destination AS number and the AS number imported or exported. Instead of single AS numbers, AS-SETs can be referenced in these attributes, which simplifies management of complex routing policies.