Guide
Classifying autonomous system numbers: private, reserved and public
An autonomous system number (ASN) identifies a network in BGP, the protocol that glues the Internet together. They look like ordinary integers, but the range a number falls in says whether it may be used on the public Internet at all. The page classifies numbers in either notation so you can validate configurations and read BGP output.
The ranges
AS numbers began as 16-bit values and were extended to 32 bits by RFC 6793. The registries divide the space as follows. 0 is reserved and must not be used in BGP (RFC 7607); a route origin authorisation naming AS 0 means “do not route”. 1 to 64495 is the public range allocated by IANA to the regional registries, apart from reserved exceptions such as 23456. 64496 to 64511 are for documentation (RFC 5398), as are 65536 to 65551. 64512 to 65534 are private (RFC 6996), and so are 4200000000 to 4294967294. 23456 is AS_TRANS, the 16-bit placeholder for a 32-bit number in sessions with old routers. 65535 and 4294967295 are reserved.
Two notations
Asplain (RFC 5396) writes every number as a single integer, such as 4200000001. Asdot writes numbers below 65536 in plain form and larger ones as two parts separated by a dot: high 16 bits, then low 16 bits. So 65546 is 1.10 and 4200000001 is 64086.59905. The page accepts either as input, with or without the AS prefix, and shows the asdot form for every result in the table.
Reading the table
The page lists one number per line. Each row shows the number, its asdot form, the scope (Private, Public, Documentation or Reserved), the kind with the size of the range, and a note citing the RFC. For public numbers the note adds that being in the allocation range does not mean the number is assigned; that has to be looked up in a registry, using the whois service of the regional internet registry.
A worked example
The sample includes ten numbers. 64512 and 65001 are Private (16-bit): free to use inside an organisation but not to be advertised to the Internet. 23456 is Reserved as AS_TRANS. 4200000001 is Private (32-bit) and appears as 64086.59905 in asdot. 64500 is Documentation under RFC 5398, and 65546 (asdot 1.10) is documentation in the 32-bit range. 15169 and 13335 fall in the public range and are labelled as such. 0 and 65535 are Reserved.
Limits
The classification follows the IANA registry ranges built into the page. It does not tell you who a public number belongs to, whether it is currently announced, or what prefixes it originates; for that use a registry whois or a routing-data service. The ranges change only by RFC, but check the IANA registry if you rely on an edge case. For address blocks rather than AS numbers, use the Private IP Checker.
Where AS numbers show up
In practice you meet AS numbers in the AS_PATH attribute of BGP routes, the list of networks a route has crossed with the originating AS at the end, in peering databases and in the whois output of the regional registries. A number’s range tells you quickly what you are looking at. A private number in a path seen from the public Internet usually means a provider forgot to strip it before announcing the route, and 23456 in a path means a router that only understands 16-bit numbers was involved. Paste the numbers from a looking-glass or routing report into the page to sort them in one pass.
Related tools
- Private IP Checker classifies IPv4 and IPv6 addresses.
- Subnet Calculator works out the prefixes an AS announces.
- IP Range to CIDR turns a registry range into blocks.
- Decimal to Binary Converter shows the bits behind the asdot split.
Checked against the current version of the ASN Checker on .