Autonomous Systems, the net within the net
Autonomous Systems are self-contained IP networks, which are administrated as a unit. It is identified externally through special routers at the borders of the AS.
AS networks are operated by ISPs , web hosting agencies, big companies and by public internet exchange points. The connections between those systems can be divided into Peering and Transit connections. Through Peering two AS make an agreement to exchange their data in a self financing way. When using Transit the smaller AS (Customer) pays to the bigger AS (Provider) for its data traffic.

IANA (Internet Assigned Numbers Authority) is responsible for the administration of all Autonomous Systems. By now approximately 21.000 AS networks have been registered worldwide. The target of a structural analysis is to investigate and display the communication between the AS.

BGP, routing between Autonomous Systems
The Border Gateway Protocol (BGP) is the de facto standard in routing protocols between Autonomous Systems. It belongs to the Path Vector Protocols; it administrates a routing table which contains the complete path to the specified target network. All AS that are passed on the way to the target are listed. On the basis of the paths within the routing tables of BGP, important information about the quantity of the AS can be collected.

The University of Oregon conducted the Route Views Project, which provides most of the required routing information for the structural analysis.

Classless Inter-Domain Routing, no route without target
At the end of the route, there’s always an IP address. This IP address is used to refer to the communication node on the IP Layer and is always dedicated to an AS. The Classless Inter-Domain Routing (CIDR) was introduced to use the 32-Bit address space more efficiently. Classless means that CIDR can be used for all address classes. CIDR makes it possible to address multiple sub networks which are connected to one ISP with a common IP prefix.