ASN (BGP) Validator

ASN and BGP: how the internet talks to itself

An Autonomous System (AS) is a collection of IP networks under a single technical administration that presents a consistent routing policy to the rest of the internet. Each AS is identified by a globally unique Autonomous System Number (ASN). ASNs are the vocabulary of BGP — Border Gateway Protocol, defined in RFC 4271, which is how ISPs, datacenters, CDNs and large enterprises announce which prefixes they originate and which paths they prefer.

This validator parses an input like AS15169, 15169 or AS-google, strips the prefix, checks that the number falls inside an officially allowed range, and tells you whether it is public, private, reserved or for documentation. It does not query any registry — everything runs locally in your browser.

2-byte vs 4-byte ASN

Originally ASNs were 16-bit integers, allowing only 1-65535. By the late 2000s the pool was nearly exhausted, so RFC 4893 (2007) and the consolidated RFC 6793 (2012) extended the field to 32 bits, lifting the cap to 4 294 967 295. Modern BGP routers negotiate the 4-octet AS capability at session setup; legacy peers see a placeholder ASN 23456 ("AS_TRANS") whenever a 4-byte number must traverse them. Any production BGP deployment in 2025 must support 4-byte ASNs natively.

Reserved and special ranges

• 0 — "none", forbidden on the wire (RFC 7607).
• 23456 — AS_TRANS, compatibility placeholder for 2-byte-only speakers.
• 64496-64511 — reserved for documentation and example code (RFC 5398).
• 64512-65534 — private use, 2-byte range. Used in internal MPLS and confederations; must never be announced to the public internet.
• 65535 — last 16-bit value, reserved.
• 65536-65551 — reserved for documentation in 4-byte space (RFC 5398).
• 4200000000-4294967294 — private use, 4-byte range (RFC 6996). Preferred today for new internal deployments.
• 4294967295 — last 32-bit value, reserved.

Allocation hierarchy: IANA, RIRs and NIRs

IANA delegates ASN blocks to the five Regional Internet Registries: ARIN (North America), RIPE NCC (Europe and Middle East), APNIC (Asia-Pacific), LACNIC (Latin America and Caribbean) and AFRINIC (Africa). LACNIC further delegates to National Internet Registries — in Brazil, that role is played by NIC.br through Registro.br. Famous public ASNs: AS15169 Google, AS32934 Meta/Facebook, AS13335 Cloudflare, AS16509 Amazon, AS6939 Hurricane Electric. Brazilian examples: AS28579 NIC.br, AS26162 Locaweb, AS262202 IX.br Sao Paulo, AS27699 Telefonica Brasil.

Day-to-day use: peering, IX and looking glasses

Network operators advertise their ASN on PeeringDB so other networks can negotiate interconnection. At an Internet Exchange Point — IX.br/PTT.br in Brazil, run by NIC.br, is one of the largest in the world by traffic — each member announces its ASN over a shared fabric, dramatically cutting transit costs. To debug routes, engineers use BGP looking glasses such as bgp.he.net, RIPE Stat or Team Cymru ASN lookup, which show how an ASN is seen from many vantage points.

BGP hijacking and RPKI

Because BGP trusts every announcement by default, a misconfigured or malicious AS can claim to originate a prefix it does not own — a BGP hijack. The textbook case is Pakistan Telecom (AS17557) rerouting all of YouTube (AS36561) globally in 2008 by accident. The defense is RPKI — Resource Public Key Infrastructure: holders cryptographically sign Route Origin Authorizations stating which ASNs may originate which prefixes, and routers drop invalid announcements. Major Brazilian providers and NIC.br itself signed nearly all their prefixes by 2024.

FAQ

Should I assign a 2-byte or 4-byte ASN today? 4-byte is now the default at LACNIC and other RIRs because the 2-byte pool is exhausted. Every BGP speaker manufactured in the last decade speaks 4-byte natively.

Can I use a private ASN on the public internet? No. Ranges 64512-65534 and 4200000000-4294967294 are private; routers at the edge of any well-run network filter them out, just as they filter RFC 1918 addresses.

How do I look up an ASN for free? https://bgp.he.net by Hurricane Electric, https://stat.ripe.net by RIPE NCC and whois -h whois.cymru.com by Team Cymru are all free and authoritative. Registro.br offers Brazilian-context data.

What does AS_TRANS (23456) do? It is the compatibility placeholder that 4-byte-capable routers use when sending an AS_PATH through a peer that only understands 2-byte ASNs. The original 4-byte number is preserved in a separate AS4_PATH attribute.

Does this tool verify that an ASN is actually allocated? No. It only confirms the number is well-formed and falls inside a legitimate public or private range. To confirm allocation and ownership, query a RIR WHOIS or RDAP service.