Every AS needs a unique identifier called an Autonomous System Number (ASN).
ASN ranges:
- to : Public -bit ASNs (assigned by RIRs)
- to : Reserved for documentation
- to : Private use (not routed globally)
- : Reserved
- to : -bit ASN space (extends the range)
Who assigns ASNs? Regional Internet Registries (ARIN, RIPE, APNIC, etc.) allocate them. You need an ASN if you plan to connect to multiple ISPs or run BGP.
Looking up ASNs: Tools like whois or bgp.he.net show which organization owns an ASN and what prefixes they announce.