mod_asn is an Apache module doing lookups of the autonomous system (AS)[1], and
the network prefix[2] which contains a given (clients) IP address.

It is written with scalability in mind. To do high-speed lookups, it uses the
PostgreSQL ip4r datatype[3] that is indexable with a Patricia Trie[4] algorithm
to store network prefixes. This is the only algorithm that can search through
the ~250.000 existing prefixes in a breeze.

It comes with script to create such a database (and keep it up to date) with
snapshots from global routing data - from a router's "view of the
world", so to speak.

Apache-internally, the module sets the looked up data as env table variables,
for perusal by other Apache modules. In addition, it can send it as response
headers to the client.