An autonomous system or AS for short is a collection of routers which share the same routing policy. What this means is that all the routers have the same view of the Internet and agree about the best way to reach any particular destination. The routers in an autonomous system are normally owned and managed by the same group of people and typically each organization will have one autonomous system. There are some cases where an organization might have more than one AS but that's quite unusual. Each autonomous system is identified by a unique autonomous system number or ASN. ASNs were originally 16-bit numbers but they have now become 32-bit in addition to public ASNs there are ranges reserved for examples in documentation and ranges reserved for private use. These can be used when two ASes want to exchange information with BGP but that information doesn't need to propagate to the rest of the Internet. These days AS numbers are shown as a single decimal number which is called the AS plain format. When 32-bit ASNs were first introduced they were sometimes shown as two 16-bit numbers separated by a dot called the AS dot format but that's rarely seen now. It's also rare to see the special ASN 23456 which was a transition measure for software which didn't understand 32-bit ASNs. So if you want to connect your network to the internet with BGP you'll need your own AS number. Autonomous system numbers are distributed by the regional internet registries. They're also available from upstream service providers who are members of one of the registries. The entire 16-bit ASN pool has been assigned to the registries already. Around 42,000 of these 16-bit AS numbers are visible on the global Internet today at the time of this recording. Each registry has also received a block of 32-bit AS numbers. Out of 29,000 assignments made by the registries around twenty three thousand eight hundred of these are visible on the global Internet. To get a complete list of the AS number delegations that have been made to the registries and to end users you can have a look at the URL displayed on the screen. Except in a few very limited scenarios in order to use BGP on the Internet you must have your own IP address space. So in fact you'll need three things from your RIR: a block of IPv4 addresses, a block of IPv6 addresses and an autonomous system number.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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