An autonomous system (AS) is essentially a network that has different policies than the networks or service providers to which it is connected. The network consists of a group of connected Internet protocol routing prefixes controlled by a network operator. In order to be accepted as an autonomous system, the network must prove that it has a clearly defined routing policy. The AS is then granted an autonomous system number (ASN) to identify it.
The format and rules for exchanging messages between computing systems is called a protocol. A protocol describes the syntax and vocabulary to be used in the communication, as well as how the communications will be synchronized. It may also lay out rules for signal authentication and error detection. The Internet Protocol (IP) is the protocol that makes up most of the Internet.
IP is responsible for moving packets of information between computer networks. An IP routing prefix or IP address is used to send these packets from a source network to a destination network in the same way that a mailing address ensures that a letter reaches the proper recipient.
Like a postal service, a group of IPs may have rules about how the information must be packaged, sorted, and routed in order to be sent. If the rules meet certain criteria, the group of IPs can be assigned an autonomous system number and become a public autonomous system. First, the AS must have an internal routing plan that is clear to other autonomous systems. Second, other autonomous systems should be able to clearly tell which networks are reachable through the AS in question. Finally, the AS should be connected to more than one other AS.
There are three different categories of autonomous systems. The multihomed AS maintains connections to multiple service providers. This allows the user to have a policy different from that of the service providers. Transit autonomous systems provide connections to other networks. Internet service providers are an example of this type of system.
The final type of system is not a true autonomous system. Is connected to only one other AS and should technically fall under the AS of the service provider. A stub can only become its own autonomous system if its policies are sufficiently different from those of the service provider.
ASNs are assigned by regional Internet registries. These registries get sets of numbers from the Internet Assigned Numbers Authority (IANA). They then assign numbers to local groups from these sets. Current assignments are listed on the IANA website.