What Is an ASN? Autonomous Systems and Internet Routing

1. The Internet as a Network of Autonomous Systems

The global internet is not a single unified network—it is a federation of more than 75,000 independent networks called Autonomous Systems (AS). Every broadband ISP (such as Reliance Jio AS55836 or Bharti Airtel AS9498), transit carrier (such as Tata Communications AS4755), content delivery network (such as Cloudflare AS13335), and university or enterprise with multi-homed routing operates at least one Autonomous System.

Inside its own network, an operator uses an Interior Gateway Protocol (like OSPF or IS-IS). Between separate Autonomous Systems—for example, at an Indian Internet Exchange Point like NIXI—routers speak Border Gateway Protocol (BGP), tagging every advertised IP prefix with an AS_PATH list of Autonomous System Numbers.

2. Public vs. Private ASN Ranges

Just like RFC 1918 reserves private IPv4 addresses, the IETF reserves private Autonomous System Numbers (RFC 6996) for internal BGP routing that must never leak onto the public internet:

• 16-bit Private ASN range: 64512 – 65534
• 32-bit Private ASN range: 4200000000 – 4294967294
• Documentation ASN ranges (RFC 5398): 64496 – 64511 and 65536 – 65551

Use our ASN Lookup utility to inspect any public Autonomous System Number in the IANA/RIR RDAP registry.