Autonomous System Number (AS Number)

What is an ASN?

An Autonomous System Number (AS number or just ASN) is a special number from 1 to 65,535 that uniquely identifies an autonomous system. This autonomous system number is required if you are to run BGP. The ASN must be unique so that IP address blocks appear to come from a unique location that BGP can find and route to.

Public and Private ASN

There are public ASN and private ASN. RFC 1930 outlines which are private and which are public.

PUBLIC ASN (1 - 65511)

The ASN in the public range are globally unique and may be announced on the global Internet. ASN are used to uniquely identify networks or systems of networks which appear to the outside world to be running a single consistent routing policy. Prefixes are 'seen' to originate from these public ASN by the exterior gateway routing protocol. This ensures that routes lead back to a unique source of a given range of IP addresses.

PRIVATE ASN (65512 - 65535)

The private ASN should not be seen on the global Internet (they shouldn't be announced via your exterior gateway routing protocol). Private AS numbers are used by ISP's who use BGP confederations. Private AS numbers are also used to provide an AS number to customers with multiple connections who have no connections to any other Internet service provider.

 

Obtaining an ASN

An AS-number must be obtained (purchased) from one of the Regional Internet Registries (RIR's). Which registry you obtain your AS number from is based upon where in the world your network resides physically and will be connecting to. You must apply to the RIR to obtain an ASN. ASN's are usually provided for a 'container fee' which is a fancy way of saying that they will charge you money to open an account and place the AS number under your account. You will need to use a form or template to request your AS number (ASN).

Here's where to obtain the template:

• AMERICAS/AFRICA - American Registry for Internet Numbers (ARIN)
  • ARIN ASN Request Template
• EUROPE - Reseaux IP Europeens (RIPE)
  • RIPE ASN Request Form
• ASIA - Asian Pacific Network Information Center (APNIC)
  • APNIC ASN Request Form
• Caribbean/Latin America - Latin American and Caribbean Internet Adresses Registry (LACNIC)
  • LACNIC ASN Request Template

Requirements

There are a limited number of ASN available, so the RIR's are very selective about whom they grant an ASN to. You will need to demonstrate the following:

1. That you have a connection to more than one ISP (or will in the next 30 days). This is called being 'multi-homed'.
   1. You will be asked to specify the exterior gateway protocol used to communicate with your ISP (this is usually BGP)
   2. You will be asked to provide the AS numbers of your ISP's
   3. You may be asked to provide the IP addresses of your ISP's routers to which you connect.
   4. You and your ISP must already have an identity on file in the RIR's database
2. You must have blocks of IP addresses that need routing.
3. You must demonstrate a need to utilize BGP with an organizationally unique route policy

 

 

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