ASN & BGP
What is BGP?
BGP (Border Gateway Protocol, RFC 4271) is the routing protocol that holds the Internet together. It runs between autonomous systems and exchanges reachability — which IP prefixes can be reached, and through which paths. The sum of all these announcements is the global routing table, also called the "full view".
What is an ASN?
An Autonomous System (AS) is a network, or group of networks, run under a single routing policy — typically an ISP, hosting provider or large organisation. Each AS is identified by an Autonomous System Number (ASN). ASNs were originally 16-bit (0–65535); since RFC 6793 the space is 32-bit (0–4294967295), written for example as AS64500.
Reserved & special ASNs
| Range | Purpose | Reference |
|---|---|---|
| 0 | Reserved tech | RFC 7607 |
| 23456 | AS_TRANS (16↔32-bit transition) tech | RFC 6793 |
| 64496–64511 | Documentation & samples tech | RFC 5398 |
| 64512–65534 | Private use (16-bit) private | RFC 6996 |
| 65535 | Reserved tech | RFC 7300 |
| 65536–65551 | Documentation & samples (32-bit) tech | RFC 5398 |
| 4200000000–4294967294 | Private use (32-bit) private | RFC 6996 |
| 4294967295 | Reserved tech | RFC 7300 |
Allocation to the RIRs
IANA hands out AS-number blocks to the five RIRs, which in turn assign individual ASNs to networks. Both 16-bit and 32-bit ranges are distributed in blocks. The full, current block-to-RIR mapping is the authoritative IANA Autonomous System Numbers registry; live per-RIR holdings are published in the RIR delegated statistics.
| RIR | Service region | Site |
|---|---|---|
| RIPE NCC | Europe, Middle East, Central Asia | ripe.net |
| ARIN | North America | arin.net |
| APNIC | Asia-Pacific | apnic.net |
| LACNIC | Latin America & Caribbean | lacnic.net |
| AFRINIC | Africa | afrinic.net |
Explore real autonomous systems and their announced prefixes on our BGP Full View.