An Autonomous System (AS) is a network run under a single routing policy, identified by a globally unique number (e.g. AS174 = Cogent, AS3215 = Orange). It is the identity block of Internet routing: with your own AS and provider-independent (PI) addresses, you announce your prefixes in BGP to any carrier.
Concrete benefits: independence (change carrier without renumbering), resilience (multi-homing across 2+ transit providers), portability (move data centers and keep your IPs). In Europe, assignment goes through the RIPE NCC, directly (LIR status) or via a sponsoring LIR. Essential hygiene: up-to-date IRR objects and signed RPKI ROAs, without which a growing number of carriers will filter your announcements.
Our AS search shows where each operator is present, data center by data center.