Explore the global registry of Autonomous Systems. An AS number (ASN) uniquely identifies a network that controls a specific set of IP addresses and maintains its own routing policy.
What is an Autonomous System?
An Autonomous System is a connected group of IP networks under the control of one or more network operators with a common routing policy. Every network that participates in BGP routing needs an AS number.
ASNs are assigned by Regional Internet Registries (RIRs):
- ARIN - North America
- RIPE NCC - Europe, Middle East, Central Asia
- APNIC - Asia Pacific
- LACNIC - Latin America and Caribbean
- AFRINIC - Africa
AS Number Ranges
- 1-65535 - 16-bit ASNs (original range, mostly allocated)
- 65536-4294967295 - 32-bit ASNs (extended range, 4-byte ASNs)
- 64512-65534 - Reserved for private use
- 23456 - Reserved for AS_TRANS (4-byte ASN transition)
Major AS Numbers
Some of the largest autonomous systems by prefix count and traffic:
- AS15169 - Google
- AS13335 - Cloudflare
- AS16509 - Amazon (AWS)
- AS8075 - Microsoft
- AS32934 - Facebook (Meta)
- AS14618 - Amazon
- AS20940 - Akamai
How to Use AS Lookup
Enter an AS number in the search box (e.g., AS15169 or just 15169) to view:
- Organization name and contact info
- All announced IPv4 and IPv6 prefixes
- Upstream providers and downstream customers
- BGP path analysis and routing policy
- Peering relationships at IXPs
AS Macros
AS-SETs (also called AS macros) group multiple AS numbers for routing policy. They enable ISPs to express complex routing relationships in a maintainable way.
FAQ
How do I get an AS number?
Request one from your Regional Internet Registry (ARIN, RIPE, APNIC, LACNIC, or AFRINIC). You need to demonstrate technical need and typically need to be multihomed or plan to peer at an IXP.
What's the difference between 16-bit and 32-bit ASNs?
16-bit ASNs (1-65535) were the original format. As these became scarce, 32-bit ASNs (up to 4.3 billion) were introduced in 2007. Both function identically in modern BGP.
Can I see which ASN an IP address belongs to?
Yes, look up the IP address to see its origin AS - the network announcing that prefix via BGP.
What is an AS-SET?
An AS-SET (or AS macro) is a named group of AS numbers used in routing policy. It lets ISPs define customer cones and peering relationships without listing every individual ASN.