What is an ASN? Autonomous system numbers explained

An autonomous system number, ASN, identifies a network that routes its own address space on the internet under one routing policy. Networks tell each other which addresses they can reach over BGP, and every route carries the ASNs it has passed through, so the ASN is how the rest of the internet sees your network. The regional internet registries assign them; whether you need one of your own is in do you need your own ASN?

What an autonomous system is

RFC 1930 defines it as a connected group of IP prefixes, run by one or more network operators, with a single, clearly defined routing policy. In practice:

• Your ASN is the origin of the routes for your address space: the last number in each route's path.
• A ROA names it. Route origin validation checks that the ASN announcing a prefix is the one its holder authorized (RPKI and ROAs).
• Route objects name it too, in the routing registries that providers build their filters from (route objects and RADb).
• The registry holds an aut-num object for it, with its contacts and, at the RIPE NCC, its routing policy.

The numbers

AS numbers were 16 bits, up to 65,535. RFC 6793 extended them to 32 bits, up to 4,294,967,295, and the registries now hand both out from one pool: the RIPE NCC and APNIC have done so since 1 January 2010. A 32-bit ASN is usually written as one number, such as 4200000000; an older notation writes it as two halves with a dot.

Some numbers are not for use on the public internet:

When a network needs one

• Two or more upstreams, with a routing policy of your own: you announce your prefixes to each over BGP.
• Peering at an internet exchange.
• Announcing your own space from your own ASN at a cloud or hosting provider that allows it; many announce it from theirs instead (the BYOIP guides).

A network with one upstream usually does not: the provider announces its prefixes from the provider's ASN.

How each registry hands them out

RIPE policy proposal 2026-01 would let a holder of RIPE NCC space have its first ASN without the multihoming test; it is in review until 10 November 2026. Each registry's page has the rest: the RIPE NCC, ARIN and APNIC.

What comes with an ASN

• BGP sessions with each upstream or peer, and their route filters.
• ROAs naming your ASN for each prefix you announce, and optionally an ASPA listing your upstreams, which the three registries now support.
• Route objects, and an as-set if others announce routes through you.
• A PeeringDB record, which exchanges and peers look up.
• An abuse contact the registry validates.

Where RTM Networks fits

We sponsor AS numbers in the RIPE NCC region: you get an ASN of your own without a RIPE NCC membership, and we handle the registry side. See PI and ASN sponsorship.