An Autonomous System (AS) is one or more connected networks that use the same routing protocol, and appear to be a single unit to any externally connected networks. For example an ISP may have a number of customer networks connected to it, but to any networks connected externally to the ISP it appears as one system or AS. An AS may also be referred to as a routing domain.

It should be noted that while OSPF routing takes place within one AS, the only part of OSPF that deals with the AS is the AS border router (ASBR).

There are multiple types of AS defined by how they are connected to other ASes. A multihomed AS is connected to at least two other ASes and has the benefit of redundancy — if one of those ASes goes down, your AS can still reach the Internet through its other connection. A stub AS only has one connection, and can be useful in specific configurations where limited access is desirable.

Each AS has a number assigned to it, known as an ASN. In an internal network, you can assign any ASN you like (a private AS number), but for networks connected to the Internet (public AS) you need to have an officially registered ASN from Internet Assigned Numbers Authority (IANA). ASNs from 1 to 64,511 are designated for public use.

The main factors in deciding if you need your own AS or if you should be part of someone else’s are:

You should not create an AS for each prefix on your network. Neither should you be forced into an AS just so someone else can make AS-based policy decisions on your traffic.

There can be only one AS for any prefix on the Internet. This is to prevent routing issues.

In addition to overseeing IP address allocation and Domain Name Systems (DNS), the Internet Assigned Numbers Authority (IANA) assigns public AS numbers. The public AS numbers are from 1 to 64,511. The ASNs 0, 54272–64511, and 65535 are reserved by the IANA. These ASNs should not be used.

ASNs are assigned in blocks by the Internet Assigned Numbers Authority (IANA) to Regional Internet Registries (RIRs) who then assign ASNs to companies within that RIRs geographic area. Usually these companies are ISPs, and to receive an ASN you must complete the application process of the local RIR and be approved before being assigned an ASN. The RIRs names and regions are:

AS numbers from 64512 to 65534 are reserved for private use. Private AS numbers can be used for any internal networks with no outside connections to the Internet such as test networks, classroom labs, or other internal-only networks that do not access the outside world. You can also configure border routers to filter out any private ASNs before routing traffic to the outside world. If you must use private ASNs with public networks, this is the only way to configure them. However, it is risky because many other private networks could be using the same ASNs and conflicts will happen. It would be very much like your local 192.168.0.0 network being made public — the resulting problems would be widespread.

In 1996, when RFC 1930 was written only 5,100 ASes had been allocated and a little under 600 ASes were actively routed in the global Internet. Since that time many more public ASNs have been assigned, leaving only a small number. For this reason 32-bit ASNs (four-octet ASNs) were defined to provide more public ASNs. RFC 4893 defines 32-bit ASNs, and FortiGate units support these larger ASNs.