What is an ASN?

ASNs are Autonomous System Numbers or AS Numbers. These are the numbers used by networks that manage their own internet connectivity. AS Numbers identify the networks announcing IP addresses using the BGP protocol. This is the routing protocol that glues to internet together.

If an IPv4 address is like a street address, then an AS Number is like a postal code for a town.

• AS Numbers are generally prefixed with AS, such as AS64512.
• An AS Number is used to describe a routing policy. Most networks have a single routing policy and so only need one AS Number. Some large networks have complex routing policies. For instance, international networks might have a per-country policy. In some cases they can have multiple AS Numbers and connect them into a BGP Confederation.
• Only 76,424 AS Numbers were visibly used at the end of September 2024. As there are over 4 billion AS Numbers available, there is no immediate danger of running out.
• Every 16-bit AS Number – the ones under 65,535 – has 256 multicast addresses available to it. These are in the 233.0.0.0/8 block.

There are about 4.3 billion IPv4 addresses but only 3.7 billion are available for ordinary use. This is less than one for each person on the planet. That is the protocol’s biggest problem.

AS Numbers can be transferred between organizations. Some people prefer a small number for their network’s AS Number. But the larger AS Numbers – those from 65552 to 4,199,999,999 – have been in use since 2007. The equipment and software that did not support larger AS Numbers has now been replaced.

Explore More Related to IPv4 Addresses:

• What is a Regional Internet Registry?
• What is RPKI?
• Addressing Modes, Multicast