What is an ASN?
An autonomous system number (ASN) is a unique number that identifies every network on the internet. An autonomous system (AS) is a group of IP networks that is managed by one or more network operators and is subject to a single and clearly defined external routing policy.
Why do I need an ASN?
If you wish to connect to other networks via an internet node and exchange data (peering), you will need an ASN. Every internet service provider (ISP) needs an ASN of its own, but individual organisations connecting to the internet via an ISP need one too.
An AS number actually forms the foundation of the internet, which is made up of various autonomous systems that communicate and exchange data with one another.
How can I apply for an ASN?
A finite number of AS numbers are available and the issuing of AS numbers is carried out by the Regional Internet Registries (RIRs) and is recorded by the Internet Assigned Numbers Authority (IANA). In principle, each provider will receive a single AS number, unless it is an extremely large provider operating on a global scale. The IANA allocates blocks of AS numbers to the RIRs. The RIPE NCC is the RIR for Europa and the Middle East. In addition to the RIPE NCC, there are four other RIRs: APNIC (Asia and Australia), AFRINIC (Africa), LACNIC (Latin America) and ARIN (North America).
Public versus private AS numbers
A public AS number is only required if an AS exchanges routing information with other autonomous systems on the internet.
A private AS number can be used if an AS is only required in order to communicate with a single provider by means of the Border Gateway Protocol. As the routing policy between the autonomous system and the provider will not be visible on the internet, a private AS number can be used for this purpose.
Is my organisation eligible for a public AS number?
Membership of a RIR is required in order to obtain a public AS number. Ownership of public objects (IPv4 and/or IPv6 addresses) is also an essential requirement in order to be entitled to a public AS.