What Is an ASN (Autonomous System Number) and Does Your Company Need One?

If your own business has its own network infrastructure, multiple internet connections, or is moving to colocation or cloud environments, you may have encountered the term ASN. It's a technical-sounding term for good reason, but understanding it and whether it applies to you and your business will save you time, money and headaches when it comes to network redundancy, reliability and reach.

What Is an ASN?

An Autonomous System Number (ASN) is a numerical identifier for a particular network (or set of networks) on the Internet that are run under a common routing policy. In other words, the Internet is a collection of networks, each of which is considered an autonomous system (AS) and needs to have a unique number so all the routers in the world can find it.
Autonomous System Numbers are used in the Border Gateway Protocol (BGP), which is the standard for defining how traffic moves between networks on the Internet. Having your own ASN means you can advertise (announce) your IPs to the rest of the Internet and control how traffic is permitted in and out of your network, rather than relying only on the BGP of your upstream provider.

ASNs come in two basic types:

• Public ASNs: A publicly visible number assigned by a regional internet registry (RIR) such as ARIN (North America), RIPE NCC (Europe), APNIC (Asia-Pacific), LACNIC (Latin America) or AFRINIC (Africa). These are used for networks with their own routing policy connected to the public Internet.

• Private ASNs: Used internally by a network or between a company and a single service provider not visible on the global Internet.
The original range for ASN values was 1 to 65,535 (16-bit ASNs). These numbers were insufficient to meet the global demand and 32-bit ASNs were adopted, allowing a much larger pool of unique values (up to 4 billion).

How Does an ASN Work?

Most large routers on the Internet need to know how they can reach every other network. Rather than looking at every individual IP address, routers use ASNs to group blocks of IP addresses together and select where traffic going to those IPs should be directed.

Here's what happens when your organization has its own ASN:

1. You are allocated one or more blocks of IP space (or bring your own).
2. You announce ("advertise") those IPs to the Internet using BGP.
3. Other networks and Internet service providers see your ASN and send the appropriate traffic your way.
4. You control your own traffic routing policy - which providers carry your traffic and how. This is different than a traditional business Internet connection where your ISP does all your traffic routing on your behalf.

Who Needs an ASN?

Most businesses don't need an ASN in fact many small and medium businesses operate fine by not having them and allowing their IPs and routing policies to be managed by their Internet Service Provider. The main reason to acquire an ASN or the motivation, for some businesses would be if they were in one of the following categories.

1. Multihomed (Have Connections with More Than One ISP)
   If your company has more than one Internet connection because you want to have redundancy (should one ISP fail, you'll still have Internet connectivity), you need an ASN and an independent IP address space because failover with standard BGP doesn't work. In other words, you can't "fail over" from one provider to another without an independent numbering scheme.

2. Want Full Control Over IP Address Space
   A company would want an ASN if they needed to make their own IPs (their own addresses) portable. That is, they want to be able to change ISPs without renumbering their entire infrastructure, which would typically require returning the IPs they had with their former ISP.

3. Run a Data Center, Hosting Company or Cloud Service
   These kinds of businesses host or resell hosting services to other businesses and therefore need their own ASN to manage and route large blocks of IPs independent from a single provider.

4. Peering Directly With Other Networks
   Large companies, ISPs and content providers sometimes peer directly with each other to avoid routing traffic through other networks, which reduces latency. This requires an ASN.

5. Want Improved Network Performance / Redundancy
   A company that owns an ASN can optimize the way in which they receive and send traffic out on their network, which can help with latency-sensitive applications, large SaaS providers or companies with a very international audience.

When You Don't Need an ASN:

Your company has a single Internet connection provider (ISP).
You don't manage your own public-facing IP address ranges.
Your infrastructure is mostly in the cloud (AWS, AZURE, Google Cloud etc.) and uses their networking software rather than your own advertised IP space.
Redundancy is handled at the application level rather than using multihomed routing.
For most companies, however, the overhead associated with an ASN far outweighs the benefits unless they're in one of the categories above.

How to Get an ASN

If your organization decides they need one, the general steps are as follows:

1. Determine eligibility, most RIRs require that you have a network that will connect to multiple external networks (multihoming).
2. Apply with the appropriate Regional Internet Registry (ARIN, RIPE NCC, APNIC, LACNIC or AFRINIC) based on your geographic location.
3. Get IP address space from the RIR or from an upstream provider that you will advertise under your ASN.
4. Configure BGP with your ISPs to start advertising your routes.
5. Maintain your registration with the RIR, including any fees and updates to routing information or contact info.

ASN Takeaway

An ASN is a basic building block of the Internet and a way to tell networks apart, allowing them to control how they're routed on the Internet. For most small businesses, which typically only have a single ISP connection, it's not a necessity. On the other hand, companies that are multihomed, want full control of their IP address space, need to peer directly with other networks or are running hosting/cloud infrastructure would benefit from having their own ASN. It's worth speaking with a network engineer and/or your ISP to see if it's a good fit based on the growth and needs of your business.