Autonomous system and autonomous system number

Autonomous system and autonomous system number

A group of networks that are under the control of a single administrator is called as an Autonomous System. All the networks must be controlled by a single entity.

Large organizations and Internet Service Providers are best examples of Autonomous Systems. The data packets in a Autonomous System are forwarded with the help of Interior Gateway Protocols (IGP). RIP, EIGRP, and OSPF etc. are the examples of Interior Gateway Protocol (IGP).

If the data packets have to be transferred between two different autonomous systems, Exterior Gateway Protocol (EGP) is used. Border Gateway Protocol (BGP) is the best example of Exterior Gateway Protocol (EGP). Border Gateway Protocol (BGP) forwards the packets between two different Autonomous Systems.

There are four types of autonomous systems and are as described below:

a. Multihomed Autonomous System: The Multihomed autonomous system is connected with more than one autonomous system.

b. Stub Autonomous System: In a stub autonomous system an autonomous system is connected only to one other autonomous system.

c.Transit Autonomous System: This autonomous system provides connections through itself. If network A wants to communicate with network C, it can either directly communicate with network C or the data can be passed from network A to network C through network B.

d.Internet Exchange Point Autonomous System: A physical infrastructure which helps the ISP (Internet Service Provider) to exchange data among their networks and are usually transparent.

Autonomous System Number (ASN) with Example

Each Autonomous System is allocated with a Autonomous System Number (ASN). The Autonomous System Number is a unique number and is used when the data packets are transferred between two different Autonomous Systems. Similar to public and private IP addresses, there are public and private autonomous system numbers are present.

Private autonomous system numbers are used when the communication between two autonomous systems is private (i.e. internet is not used as a communication link between two autonomous systems).

 When packet has to be forwarded from one autonomous system to other autonomous systems over the internet, public autonomous system numbers are used.

Autonomous System number was defined by 16 bits until 2007. Hence 216 autonomous system numbers can be generated. ASN 0 and ASN 65,535 are reserved numbers. The autonomous systems numbers between the range 64,512 to 65,534 are used as private autonomous system numbers.

 These can be used where internet is not used for communication. After 2007 32 bit Autonomous System Numbers came in to existence.

IANA (Internet Assigned Numbers Authority) assigns block of autonomous system numbers to RIR (Regional Internet Registries). RIR assigns autonomous numbers to the entities within its area from the block give by IANA.

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