When do you need a public ASN?
You need a public ASN when you run a multi-homed network, meaning you connect to more than one provider and want to control your own routing and keep your IP space if you switch providers. You request one from your regional internet registry (ARIN, RIPE NCC, APNIC, LACNIC, or AFRINIC) or through an upstream provider. A single-homed network usually does not need one.
How to check an ASN
To see the ASN behind any IP address, and whether it belongs to an ISP, hosting provider, or data center, use the ASN Lookup tool. For programmatic checks and risk flags, Abstract's IP Intelligence API returns the ASN, its type, and signals such as is_hosting and is_proxy for any IP.
Frequently Asked Questions
What is the difference between a public and private ASN?
A public ASN is globally unique and routes traffic on the public internet. A private ASN is for internal use only and never appears in the global routing table.
What is the private ASN range?
RFC 6996 reserves 64512 to 65534 for 2-byte private ASNs and 4200000000 to 4294967294 for 4-byte private ASNs.
What is the difference between a 2-byte and a 4-byte ASN?
A 2-byte ASN is a 16-bit number from 0 to 65,535. A 4-byte ASN is a 32-bit number from 0 to 4,294,967,295, introduced because the 2-byte pool was running low.
What is a 4-byte ASN?
A 4-byte ASN is a 32-bit Autonomous System Number ranging from 0 to 4,294,967,295. It is written as AS followed by the number, the same as a 2-byte ASN.
Do I need a public or private ASN?
Use a public ASN if you announce routes on the public internet, usually for a multi-homed network. Use a private ASN if you only run BGP inside your own network.
What is AS23456?
AS23456, called AS_TRANS, is a reserved placeholder that older 2-byte-only routers display in place of a 4-byte ASN they cannot represent.
