ASN Format Validator

Every autonomous system on the internet — a provider, a carrier, a company with its own network — carries a number identifying it in inter-network routing. Until 2007 those numbers fitted in 16 bits, and when the space ran out they were extended to 32. The extension brought a second way of writing the same number, and the coexistence of both confuses anyone reading a routing table.

Enter the number in either form and the page converts it to the other, saying which one it recognised. Dotted notation splits the number into two 16-bit halves: the value 65536 becomes 1 dot 0, because it is the first one outside the old space. The conversion is plain arithmetic — the high part multiplied by 65536 plus the low part.

RFC 5396 recommends the plain notation, without the dot, and that is what most tools and registries use today. Dotted notation survives in Cisco equipment configuration and in older documentation. Worth knowing that there are ranges reserved for private use, analogous to private IP addresses, which should not appear in public routing.