An autonomous system is one network or sets of networks under a single administrative control. An autonomous system might be the set of all computer networks owned by a company, or a college. Companies and organizations might own more than one autonomous system, but the idea is that each autonomous system is managed independently with respect to BGP. An autonomous system is often referred to as an 'AS'.
A good example is UUNet, who uses one autonomous system as their Eurpoean network, and a separate autonomous system for their domestic networks in the Americas.
"Autonomous System Numbers (ASNs) are globally unique numbers that are used to identify autonomous systems (ASes) and which enable an AS to exchange exterior routing information between neighboring ASes. An AS is a connected group of IP networks that adhere to a single and clearly defined routing policy."
To identify each autonomous system, a 'globally unique' number is assigned to them from a centralized authority (ARIN) so that there are no duplicate numbers. Globally Unique means exactly that. Within the entire Internet all around the globe, the AS number should be unique. The AS number will be from 1 to 64511, and the next highest unused number is what is generally assigned. These numbers are referred to as 'AS numbers'. The American Registry for Internet Numbers (ARIN) is the authority responsible for tracking and assigning these numbers as well as managing IP address allocations and assignments. ARIN charges a fee to organizations wishing to obtain an AS number to cover the administrative costs associated with managing AS number registrations and assignments.
To receive an AS number from ARIN, you must be able to prove you are 'dual homed' to the Internet, which means that you have more than one Internet provider with which you plan to run BGP. You must also have a 'unique routing policy' that differs from your BGP peers. Some companies have difficulty getting an AS number.
Here is a short list of the top (per Caida's Skitter Plot April 2003) ISP's system numbers. You can always go to ARIN's website to look them up.
||UUnet (U.S. domestic) (AS 701-705)|
|1239||Sprintlink U.S. Domestic|
|3561||Cable and Wireless (aq'd by SAVVIS)|
PRIVATE AS NUMBERS (64512 - 65535)
If it is not necessary to connect to the Internet, or you are part of a special type of BGP configuration you can use any of the AS numbers 64512 through 65535. However, these numbers should NOT be seen on the global Internet. One example of when you might use private AS numbers is in BGP confederations. The confederation AS number should not be seen on the global Internet.
AS NUMBERS AND BGP
BGP learns and exchanges path information regarding the route to a given destination network by keeping lists of AS numbers and associating them with destination networks. This is why AS numbers should be unique. BGP makes certain that an AS number does not appear in a path more than once, thereby preventing routing loops.