An autonomous system (AS) is a IP net, which is administered as unit and common Routing minutes used. This net again can consist of subnetworks. A AS stands under a common administrative administration, usually from an Internet service Provider (ISP), an international company or a university. Autonomous systems are among themselves connected to the Internet.

Administration

A clear AS-number (Autonomous System NUMBER, ASN) is assigned to each autonomous system. This has 16 bits a Integer value, corresponds to 65536 possible AS. Public ASN, which may be used in the Internet, lies within the range of 1 to 64511. Private one, which may be used only within an organization, within the range of 64512 to 65535. At present are assigned over 26.000 numbers. An extension on 32 bits ASN is planned.

The administration of the ASN takes over those Internet Assigned Numbers Authority (IANA). This delegates the dispatching of far to regionally the Internet Registries (RIR). These are ARIN (North America), RIPE NCC (Europe and Asia), APNIC (Asia/Pacific), LACNIC (Latin America, Karibik) and AfriNIC (Africa).

Types

Depending on whether as forms a END or a subnode in the superordinate net, one differentiates between the following AS-types:

• Stub AS are attached over exactly a left to exactly a Provider (end nodes).
• Multihomed Stub AS are for reasons of the reliability over several left to exactly a Provider attached (end nodes).
• Multihomed AS are attached for reasons of the reliability to several Provider (end nodes).
• Transit AS are tied up to other transit AS and represent the service Provider for the aforementioned three types in the form of Internet Backbone networks (subnodes).

By the allocation of the Internets into autonomous systems a better efficiency as well as a reduction of the storage location of the information necessary for the Routing are reached (hierarchical Routing).

Routing

For the Routing within a AS, the so-called Intra AS Routing, is responsible the operator, for the inter+ AS Routing between the autonomous systems, gives it uniform standards.

Inter+ AS Routing minutes are called also Exterior gateway Protocols (EGP). The EGP used at present is the Border Gateway Protocol (BGP). With BGP one converts the so-called policy based Routing, which is described further down in its own section.

Intra AS Routing minutes are called also Interior gateway Protocols (IGP). Examples are dasRouting information Protocol (RIP), the open Shortest Path roofridge Protocol (OSPF) or the Intra Domain Intermediate system ton of Intermediate system Routing Protocol (IS-IS).

Customer, Peers, Provider

With the Interdomain Routing (on one Meta level) typically between customers, Peers and Providern one differentiates:

• Another autonomous system is my customer ("„Customer "“; "„downstream "“), if it pays me money for the fact that it can exchange data over a direct line ("„left "“) with me (and over me with the remainder of the Internets).
• Another autonomous system turned around my Provider ("„upstream "“), if I pay it money for the fact that I can exchange data over a direct line ("„left "“) with him and the remainder of the Internets.
• If two autonomous systems are similarly largely, importantly, influential and well tied up, then they can agree on the fact that they divide the costs of direct lines among themselves. In this case it gives neither customer nor Provider, but one speaks of equal Peers. (This Peers should not be confounded with the Peers of a Peer ton Peer network.)
• The completely large Internet Provider, which only customer and Peers have, but nowhere the role of a customer takes, calls one also Tier-1-Provider. One calls autonomous systems, which customer is exclusive from Tier-1-Providern, also Tier-2-Provider. The affiliation to animal x could general be defined as the customers of animal (x-1); usually such distinctions are not made however.

The distinction between customers, Providern and Peers takes place only on one Meta level - in the data conveyed by Routing minutes it reflects itself only indirectly, i.e. in particular in the definition of the Routing Policys.

Policybasiertes Interdomain Routing

The fundamentals of usual Policys for passing on routing information can be summarized as follows:

• If an autonomous system is my customer, then I communicate all to it my routes, which I know: I would like to make it for my customer possible to complete as much as possible its traffic over me because I make thereby money.
• If an autonomous system is my Provider, then I communicate the routes to it to my customers, thus my customers am attainable and I at them to earn can. I do not divide my Provider however the routes to my Peers or to my other Providern with: Otherwise I would have to pay my Provider on for the data more moneys transmitted over me, however no more would not earn (with Peers) or, still more badly, would have to pay even twice (also on mine second Provider).
• Same applies to Peers: I communicate to a Peer only routes to my customers, thus my customers also about the Peer to be achieved to be able and I then no money on my Provider pay must. However I do not communicate routes to my Peer to my Provider, since he can exchange data otherwise at my expense, without I earn to it. I mostly communicate normally also no routes to my Peer to my other Peers, since he loads my network otherwise unnecessarily, without I earn to it.

As one can clarify easily, such, purely economically steered policy based Routing mostly results in ways, which technically everything else is as optimal. For example could theoretical-proves two rout with different Providern data over one rout at a common customer to exchange and would from each other far away be in this case only two Hops - however such a scenario for obvious reasons forbids itself; the customer would not permit such a choice of route, since he would suffer thereby substantial financial losses.

Examples

AS are used particularly by ISPs, international enterprises and universities

Related links

• Trace route with ASN dissolution

Articles in category "Autonomous system"

We found here 20 articles.

Related Websites

We found here 3 related websites.

• Autonomous system (Internet) - Wikipedia, the free encyclopedia
  Even though there are multiple autonomous systems supported by the ISP, the Internet only sees the routing policy of the ISP. That ISP must have a public, ...


• autonomous system - a definition from Whatis.com - see also: AS
  On the Internet, an autonomous system (AS) is the unit of router policy, either a single network or a group of networks that is controlled by a common ...


• Autonomous System AS
  AlterNet, CERFnet, SPRINTlink - all are Autonomous Systems. ... A more enlightening view is of an Autonomous System as a collection of CIDR IP address ...