AS
Autonomous System

The Internet is divvied up into a large number of separate networks, called “Autonomous Systems”, or AS.  Each AS is a distinct, separate domain, similar to a city with our U.S. Postal system.  The AS is given an ASN (AS Number) to identify it with. .  Each ISP has it’s own ASN, and some of the larger ISP’s have several ASN’s.

Autonomous Systems give order to the Internet.  They logically divide up the Internet into numbered domains, which streamlines the routing.  Cisco developed a protocol, called BGP (Border Gateway Protocol), which is used to route packets between the borders of the AS.  The current universally accepted version is BGP4.

The AS will use an “Internal” routing protocol such IGRP (Internal Gateway Routing Protocol) or IS-IS (Intermediate System to Intermediate System), so that every router in that AS knows how to get to every other router and destination within that AS.   They use BGP4 for external routing, outside the AS.

Advertising Routes - the ISP’s advertise (send out) all the IP addresses that belong to them.  Large blocks of addresses are combined within the AS, and this is called “aggregation”.  In this way, their customers do not need to singularly advertise their own routes, because the ISP does it for them.  Every IP address that you can get to on the Internet is reachable because someone, somewhere, has advertised a route that covers it.  For example, the reason that an AOL dialup user can send a packet to 10.10.20.1 (for example) is that the ISP (AS 64512) advertised that route to the two upstream providers (AS 4969 and AS 701), who in turn advertised that route to AS 690 (ANS, which provides IP service for AOL).

Common ASN’s

3561        MCI

1239        SprintLink (they also uses other ASNs, but 1239 will always appear somewhere in the AS-PATH when looking at SprintLink routes from some other provider)

701          UUNET

174          PSI

1673        ANS (the old ANS ASN, 690, should be retired by now)

1              BBN

4200        AGIS (the old Net99 ASN, 3830, should be retired by now)

4969        Net Access

There are hundreds of ASNs in use in the Internet, and thousands of ASNs in use in internal networks all over the world. If you want to take a look at live ASN info, check out http://www.merit.edu/ipma/routing_table or telnet to route-server.cerf.net, a Cisco that cerf.net loads with multiple full BGP routing tables.