What Is an ASN (AS Number)?

An ASN, or Autonomous System Number, is the unique number that identifies your network on the internet. Other networks use it to recognise you and to decide where to send traffic for your IP addresses.

You need one to announce your own IP address space with BGP, and to connect your network to more than one upstream provider.

What an autonomous system is

The internet is not one network. It is tens of thousands of networks that agree to carry each other's traffic: internet providers, hosting companies, universities, banks and large enterprises. The RIPE NCC defines an autonomous system (opens in a new tab) as "a group of IP networks run by one or more network operators", and each one is identified by a unique number, its ASN.

The routers of these networks tell each other which IP addresses they can reach, using the Border Gateway Protocol (BGP). Each announcement says "these addresses are behind AS64500" (a number made up for the example). Everyone else builds their view of the internet from those announcements. Without an ASN of your own, you have no identity to announce your addresses under.

Why people need an ASN

• To announce your own IP addresses. Addresses of your own, leased or assigned to you, are announced from your own ASN, not your provider's.
• To be multihomed. If you buy internet from two or more providers, an ASN lets you use all of the links at once and keep running if one provider fails.
• To control your traffic. With your own ASN you can choose which provider carries which traffic, and move addresses between providers without renumbering.

If you use one provider and its addresses, you do not need an ASN. The provider announces your traffic under its own.

What an ASN looks like

ASNs are written AS followed by the number, for example AS64500. The original numbers were 16 bits long, which allows about 65,000. 32-bit numbers, up to about 4.3 billion, were added later. Current equipment supports 32-bit ASNs, and the RIPE NCC has handed out 32-bit numbers by default since 1 January 2009.

Some ranges, such as 64512-65534, are reserved for private use (opens in a new tab) and are not registered to anyone, so they cannot be used on the public internet.

Who gives out ASNs

Like IP addresses, ASNs are handed out by the Regional Internet Registries. We work in the RIPE NCC region, which covers Europe, the Middle East and parts of Central Asia, and the rest of this post is about that region.

At the RIPE NCC any organisation can acquire an ASN (opens in a new tab), with a contract either with the RIPE NCC itself, as a member, or with a sponsoring LIR (see what an LIR is). The RIPE NCC's current policy is that the network must be multihomed and have a unique routing policy, and the request has to show that routing policy.

Getting an ASN through a sponsoring LIR

Becoming a RIPE NCC member just to hold an ASN means paying the membership fee and running an LIR. A sponsoring LIR makes the request for you instead, and you stay its customer, not a member.

With us:

• You do not need to become a RIPE NCC member. There is no membership fee to pay.
• The RIPE fee is included. The price is €150 a year, with no setup fee, and nothing extra to pay to the RIPE NCC.
• It takes about a week. The RIPE NCC usually assigns the number within 2 business days of our request, but the whole process, with your documents, the contract and the invoice, takes about a week from start to finish.
• You get a /48 of IPv6 (PA) space with it, free.
• It is for businesses and individuals in the RIPE NCC service region. See the list of countries.

How it works

1. You get in touch, and we ask for the information the RIPE NCC needs: your RIPE objects (or we help you create them), the IP space you will announce and your BGP peering partners.
2. We prepare the sponsorship agreement for you to read.
3. You sign it and send it back with your documents.
4. You pay the invoice by card or bank transfer.
5. We submit the request to the RIPE NCC.
6. The RIPE NCC assigns the ASN to your organisation.

What you do next

An ASN alone does not put you on the internet. You also need IP addresses to announce, upstream providers that accept your announcements, and a router that speaks BGP.

• Addresses. If you do not have your own, you can lease IPv4 or IPv6 space and announce it from your ASN. The block size calculator shows what size you need.
• Setting it up. Our guide to setting up your ASN covers routing registries, RPKI and the first BGP session.
• Help with BGP. If you would rather not do the configuration alone, see our BGP consultancy.

Common questions

Do I need an ASN to use IP addresses? No. You need one to announce addresses yourself. If your provider announces them for you, you do not.

Can I get an ASN through you as an individual? Yes, if you are at least 18 (or older, depending on where you live) and are based in the RIPE NCC region. Individuals and businesses can both use our service.

How much does an ASN cost? With us it is €150 a year, the RIPE fee included, and there is no setup fee.

How long does it take to get an ASN? The RIPE NCC usually assigns it within 2 business days of our request. From your first message to an assigned ASN it takes about a week.

Is an ASN the same as an IP address? No. An IP address identifies a device and an ASN identifies a whole network, the thing that announces a group of addresses.

Can an ASN be moved to another organisation? Yes, through a transfer that the RIPE NCC approves. We handle those too: see Resource Transfers.