Autonomous Systems & ASNs

The internet isn't one network; it's tens of thousands of independent networks that agreed to interconnect. BGP is the language they use to exchange reachability. But before two networks can speak BGP, each needs an identity: a number that says "this is me, and I set my own routing policy." That identity is the Autonomous System Number.

What is an Autonomous System?

An Autonomous System (AS) is a collection of IP prefixes under a single administrative and routing policy. The classic definition: a network (or group of networks) operated by one entity that presents a consistent, unified routing policy to the rest of the internet.

"Single policy" is the key idea. Inside an AS, the operator decides freely how packets move, which IGP runs, how routers connect. To the outside world, the AS is one black box that makes one set of decisions about what it advertises and what it accepts. Comcast, Google, your university, a regional ISP: each is one or more autonomous systems.

Every AS is identified by a globally unique AS Number (ASN). Uniqueness matters: the ASN is how every router on the internet refers to your network, so no two operators can share one.

Where ASNs come from

You don't invent an ASN; you're assigned one, through a delegation chain that keeps them globally unique:

• IANA holds the master pool of all ASNs.
• IANA delegates large blocks to the five Regional Internet Registries (RIRs): ARIN (North America), RIPE NCC (Europe/Middle East), APNIC (Asia-Pacific), LACNIC (Latin America), and AFRINIC (Africa).
• An operator requests an ASN from the RIR (or a sponsoring LIR) serving its region, justifying the need, typically that it will multihome to more than one provider.

16-bit vs 32-bit ASNs

Original ASNs were 16-bit: values 0-65535. With ~64,000 usable numbers, the pool ran low as the internet grew, so RFC 6793 introduced 32-bit (4-byte) ASNs, extending the range to 0-4294967295. The old 16-bit numbers are simply the bottom of the new 32-bit space; they remain valid.

A 32-bit ASN can be written two ways:

• asplain: one decimal number, e.g. 131072. This is the modern, preferred notation.
• asdot: a high.low pair, e.g. 2.0 (the same value). You'll still see it in older docs and some CLIs.

Public vs private ASN ranges

Like IP addresses, ASNs have public ranges (globally routable, assigned by RIRs) and private ranges (for internal use only). Private ASNs let you run BGP inside your own network without burning a globally unique number, but they must be stripped at the internet edge (e.g. with remove-private-AS), because they are not unique and have no meaning to anyone else.

AS_TRANS (23456) is a compatibility placeholder: when a route with a real 32-bit ASN passes through an old router that only understands 16-bit numbers, the path shows 23456 while the true ASN travels in a separate attribute.

The ASN is the unit of routing policy

Why does all of this matter? Because the ASN is the atom of BGP. It is the unit that policy is written around, and it's the value that shows up in the AS_PATH: the list of autonomous systems a route has traversed. Every time a route crosses from one AS to another, the sending AS prepends its number. That AS_PATH is how BGP picks routes and how it prevents loops (an AS that sees its own number in a path rejects the route). We'll dig into that mechanism in bgp-path-vector.

A quick forward look at how ASNs frame the two kinds of BGP session:

• A session between different ASNs is eBGP (external BGP), the protocol between organizations.
• A session within the same ASN is iBGP (internal BGP), carrying routes across one organization's network.

That single distinction, same AS or different AS, drives much of how BGP behaves. We'll only name it here; the full story comes in the eBGP and iBGP modules.

What's next

You now know what an AS is, how ASNs are allocated, and why every route carries them. Next, in bgp-path-vector, we'll follow the ASN into the AS_PATH and see how BGP uses that list of autonomous systems to choose routes and stop loops, the core of what makes BGP a path-vector protocol.