ASN

An autonomous system (AS) is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators on behalf of a single administrative entity or domain, that presents a common and clearly defined routing policy to the Internet. Each AS is assigned an autonomous system number (ASN), for use in Border Gateway Protocol (BGP) routing. ASNs are assigned to local Internet registries (LIRs) and end-user organizations by their respective regional Internet registries (RIRs), which in turn receive blocks of ASNs for reassignment from the Internet Assigned Numbers Authority (IANA). The IANA also maintains a registry of ASNs which are reserved for private use (and should therefore not be announced to the global Internet). Originally, the definition required control by a single entity, typically an Internet service provider (ISP) or a very large organization.

What is an ASN?

An Autonomous System Number (ASN) is a unique global identifier assigned to Autonomous Systems (AS) , the distinct networks that make up the Internet. Think of an ASN as a social security number for networks: just as each person has a unique SSN, each independently operated network needs its own ASN to be recognized and communicate on the Internet.

• Management: ASNs are managed and distributed by the Internet Assigned Numbers Authority (IANA) and the five Regional Internet Registries (RIRs).
• Format: Originally, ASNs were 16-bit numbers ranging from 0 to 65535, but as the Internet grew, a new 32-bit format was introduced, expanding the available numbers into the millions.

Practical Uses

Network operators use ASNs to establish connections, manage routing policies, and troubleshoot connectivity issues between different parts of the Internet. An ASN becomes necessary for businesses when they need to operate their own independent network with multiple connections to different Internet service providers or when they require direct control over their routing policies.

• Examples: When you see an ASN like AS7018 (AT&T) or AS8075 (Microsoft), you're looking at the official identity of that organization's network infrastructure.
• ISP Connection: Most organizations connect through their ISP's ASN rather than maintaining their own.