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Is it possible to have an autonomous system with just one IP address?

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Is it possible to have an autonomous system with just one IP address?

Theoretically, yes, but the ISPs will not advertise any IPv4 prefix larger than /24 or IPv6 prefix larger than /48 on the public Internet. You could certainly have that inside your own network or with another company or ISP, but it will not work on the Internet.

Also, the AS concept is really decoupled from the IP addressing concept. You could certainly set up a transit AS between two other ASes and use only private addresses, or one or two public addresses without advertising any of your addressing because only the two ASes for which you are providing transit services need to know only your directly connected addresses and not advertise them.

That means you could have an AS with no public addresses. The AS and its addressing do not necessarily directly relate.

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The longest prefix/smallest subnet for IPv4 that can be announced on public BGP is /24 = 256 IP addresses.

Note that each announcement likely ends up in the global routing table. Limiting the amount of fragmentation (somewhat) protects that table from exploding. Still, it holds close to 900,000 IPv4 prefixes (out of 16 million theoretically).

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    The full BGP table contains 900k routes not addresses. I believe there are around 14,461,947 possible /24 blocks in the public space. If every one routed to every other, there would be 104,573,948,284,431 possible routes, quite a lot more than 16m.
    – Charlieface
    Jul 19, 2021 at 19:28

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