untangling_the_web

DOCID: 4046925
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There is a further layer to the addressing and routing of Internet traffic that we need
to know about. As the Internet grew, it quickly became apparent that it would be
impossible for every network to know every other networks' "gateways," as they
used to be known. This works only a small scale, and the Internet is anything but
small. Thus, the idea of autonomous systems was introduced, by which only those
devices within a specific AS would know all of that AS's routes.
Autonomous System
An autonomous system (AS) is a network or group of IP networks run by a single
entity using common routing policies or protocols. Another way of looking at an AS is
to think of it as a lot of networks being handled on the Internet as one logical
domain. There are two types of Autonomous System numbers: Public AS numbers
and Private AS numbers. A public AS has a unique number (ASN) associated with it;
this number is used in both the exchange of exterior routing information (between
neighboring Autonomous Systems), and as an identifier of the AS itself. The
introduction of autonomous systems also meant that AS's could run their own
internal network any way they pleased, which made sysadmins happy. We are only
interested in Public ASNs.
In order to keep track of these new AS's, each AS was assigned an AS number (a
16 bit integer) by lANA, the Internet Authority for Assigned Names and Addresses.
lANA assigns both IP addresses and ASNs to Regional Internet Registries (the three
biggest are ARIN, APNIC, and RIPE), who then assign both types of numbers to
custorners.!"
The rules for how traffic is handed off among and between AS's on the Internet are
called protocols, and there are a number of them currently in use. The core external
routing protocol for the Internet is the Border Gateway Protocol (BGP), which we'll
look at next.
Border Gateway Protocol (BGPl
BGP works by maintaining a table of IP networks or 'prefixes' that designate network
accessibility between autonomous systems (AS).
In this example, imagine Customer 1 in Baltimore wants to reach a website
maintained by an ISP in Los Angeles, represented here by Customer 4 at Regional
ISP (AS2). Customer 1's Regional ISP (AS1) routes the request through National
146 Elquapo's Guide to Routing-Part 3, BPG, 20 August 2003,
(14 November 2006).
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