ASN lookup

What an ASN is

The internet is not one network but a collection of independently-operated ones, each identified by an Autonomous System Number, that agree to exchange routes with each other via BGP (the protocol that decides how traffic actually gets from one network to another across the internet). An ISP, a large company, a cloud provider and a university can each hold their own ASN.

What is looked up

The registry record for the AS number itself: who it is registered to, its short registry name, and the country of registration - the same RDAP infrastructure and regional registries (ARIN, RIPE, APNIC, LACNIC, AFRINIC) as the IP lookup, applied to the AS number rather than an address.

Why this is useful alongside an IP lookup

An IP address belongs to a network (an address block); an ASN identifies the network operator that announces routes for that block to the rest of the internet. Looking up both together answers "whose address is this" and "who actually operates the network it sits on" - which are related but not always the same organization (a customer's address space can be announced by their upstream provider's ASN, for instance).

Related tools

See IP address lookup for the address-block side of this.