Look up BGP Autonomous System (AS) numbers, classify their type (public/private/reserved), and learn about ASN ranges and BGP basics.
ASN Range Reference
| Range | Type | RFC |
|---|---|---|
| 0 | Reserved (zero) | RFC 7607 |
| 1 – 23,455 | Public 2-byte ASN | RFC 1771, RFC 4893 |
| 23,456 | AS_TRANS | RFC 6793 |
| 23,457 – 64,495 | Public 2-byte ASN | RFC 1771, RFC 4893 |
| 64,496 – 64,511 | Documentation 2-byte | RFC 5398 |
| 64,512 – 65,534 | Private 2-byte ASN | RFC 6996 |
| 65,535 | Reserved (last 2-byte) | RFC 7300 |
| 65,536 – 131,071 | Public 4-byte ASN | RFC 6793 |
| 131,072 – 4,199,999,999 | Public 4-byte ASN | RFC 6793 |
| 4,200,000,000 – 4,294,967,294 | Private 4-byte ASN | RFC 6996 |
| 4,294,967,295 | Reserved (maximum) | RFC 7300 |
BGP AS Types
Stub AS
Connected to only one other AS. Traffic enters and exits through a single provider. Most enterprise networks are stub ASes.
Multihomed AS
Connected to multiple other ASes but does not transit traffic between them. Provides redundancy and load balancing.
Transit AS
Carries traffic between two or more ASes. Internet backbone providers and large ISPs are transit ASes.
ASN Dotted Notation
4-byte ASNs can be expressed in dotted notation (ASPLAIN vs ASDOT). The format is HIGH.LOW where HIGH = ASN ÷ 65536 and LOW = ASN mod 65536. For example, AS131072 = 2.0 and AS196610 = 3.2. 2-byte ASNs (0–65535) are the same in both notations.
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2026