An Autonomous System Number (ASN) or AS Number is a globally unique identification of your network on the Internet, and is essential in exchanging exterior routing information with other networks.
For an Autonomous Routing implementation, an AS number is required in order to be able to route on the Internet independently, for example when connecting to an Internet Exchange for peering or buying transit services from a carrier.
How do you recognize an AS Number?
An AS number is simply a number between 1 and 64511 and is generally written as for example AS12345. You cannot simply pick a number however: it as to be officially assigned in order to guarantee global uniqueness.
In the European region, the AS number needs to be officially requested with, and assigned by, the RIPE Network Coordination Center (NCC). The RIPE NCC is one of five Regional Internet Registries (RIRs) providing Internet resource allocations, registration services and co-ordination activities that support the operation of the Internet globally.
After assignment, the AS number needs to be registered in the public RIPE database, and maintained (kept up to date) to reflect your routing policy, along with other database objects that glue the number to your company ('organization' and 'person' objects) and to the IP Space you use ('route' object).
An Autonomous System has a single and clearly defined routing policy for all IP prefixes run and originated by it on the Internet. Routing policy is defined as how routing decisions are made in the Internet. Generally, an IP prefix can belong to only one AS.
The routing policy you register in the public RIPE database is used by some parties to automatically filter (allow or deny) your routes and traffic on the Internet. It is therefor vital the data in the RIPE database is an adequate representation of your actual routing policy.
AS number in BGP
In Autonomous Routing, the Border Gateway Protocol (BGP) is the inter-Autonomous System routing protocol that is used for exchanging exterior routing information with other networks according to your Routing Policy. BGP is based on the use of AS numbers between networks. For more info on BGP look at the Internet Standard: RFC4271.
Documentation on AS numbers
Open Peering can take care of the request of an AS number (with RIPE) for you and maintain registration information for this AS number in the RIPE database:
Only in combination with IP Space
A request will only be approved by RIPE if there is also a block of (at least a /24 or 256 addresses) registered in the RIPE database on the customers name which will be routed under this AS number.
At least two peers or upstream already
Further RIPE requires there are already agreements with other networks (AS'es) two exchange routing information with this AS number via BGP (e.g. transit or peering).
IP request first, then AS
With a simultaneous AS and IP request, first the IP request will be handled, and then the AS request.
Yearly recurring fee
The yearly recurring costs are charged on the one hand to cover the costs RIPE charges Open Peering for maintenance of the RIPE registration service, and on the other hand to cover the costs of maintenance of the customers data in the RIPE database including customer requested changes and updates on that data.
Upon cancellation of the contract for an AS number with Open Peering, the maintenance of, and financial responsibility for the AS number will have to be transferred to an other RIPE approved local registree contracted by the customer. If the AS number is not transferred before the end of the contract, the AS number will be removed from the RIPE database and returned to RIPE, and cannot be used anymore.