Interconnection · Lesson 10

What does an autonomous system number identify?

An ASN (Autonomous System Number) identifies an autonomous system: a network operated under one routing policy. It gives Internet routing a way to name the administrative networks exchanging reachability information.

One network, one policy

An AS is an administrative boundary.

An autonomous system is not one device and does not necessarily mean one physical location. It is a collection of networks and routers managed according to a common routing policy.

The ASN names that system when it exchanges routes with other systems. BGP (Border Gateway Protocol) can then describe which AS originated a prefix and which AS path an announcement followed.

An ASN identifies the policy boundary, not the destination itself.The destination is described by a prefix. The ASN tells you which administrative system is involved in announcing or carrying that reachability.

Build the model

Four pieces of an Internet route

01

ASN

Names an autonomous system and its routing policy boundary.

02

Prefix

Describes the destination range of IP addresses.

03

Origin

Indicates the autonomous system that first announces the prefix.

04

Policy

Controls which routes are accepted, preferred, or shared.

Mini experiment

Read an announcement

Select the part of a route announcement you want to inspect.

ANNOUNCEMENTAdministrative identity

AS 64520 identifies the autonomous system operating under one routing policy.

The ASN names the administrative system; it does not name a single device.

Knowledge check

Test your ASN model

Answer five questions. The answer order changes each time.

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