ASN Lookup
ASN Lookup helps you lookup autonomous system number information, for routing analysis, ownership checks, and faster network troubleshooting.
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Technical Analysis & Guide
What It Does
ASN Lookup accepts either an IP address or an autonomous system number. For an IP it identifies the AS that originates the BGP route covering that address; for an ASN such as AS15169 it describes the network behind the number, including the holder name, the registry that issued it and its registered country. It also lets you tell at a glance whether a number belongs to the public, private, documentation or reserved ranges.
Why It Matters
- →Peering requests: Before configuring a BGP session you need the exact ASN, name and registry of the other side, and a typo in a 4-byte number brings up nothing at all.
- →Traffic attribution: Flow logs and DDoS reports show source IPs; mapping them to origin ASNs tells you whether an attack comes from one hosting provider or from thousands of residential networks.
- →Customer onboarding: An ISP accepting a new BGP customer must check that the customer's ASN is public, really registered to them and not a private number that would leak upstream.
- →Cloud egress clarity: Requests from a cloud VM carry the cloud provider's ASN, which explains why blocklists and geofencing treat your workload as a datacenter rather than as your company.
- →Audit of your own space: Seeing your prefixes attributed to an unexpected ASN is often the first sign of a misconfiguration at an upstream or of a deliberate hijack.
How to Read Results
- ASN: Written as AS followed by an integer (asplain); numbers above 65535 are 4-byte ASNs, which some older equipment prints in asdot form, for example 1.10 for AS65546.
- Holder or AS name: The organization name registered with the RIR, often a short handle like GOOGLE or a legal name; it does not prove who operates the routers today.
- Covering prefix (IP queries): The route in the global table that contains the address; when both an aggregate and a more-specific exist, the longest match is the one that carries traffic.
- Registry and country: Which RIR issued the number and the country of the registrant, which is an administrative fact and not where the traffic flows.
- Range classification: 64512-65534 and 4200000000-4294967294 are private, 64496-64511 and 65536-65551 are for documentation, and 0, 23456, 65535 and 4294967295 are reserved; none should appear in the public routing table as an origin.
Technical Background
An autonomous system is a set of routers and prefixes under one administration that presents a single, coherent routing policy to the rest of the Internet (RFC 1930). Its ASN is the identifier BGP uses in the AS_PATH attribute to detect loops and to let other networks express policy. The rightmost ASN in an AS_PATH is the origin: the network that injected the route. RIRs only issue public ASNs to organizations that are multihomed or have a routing policy distinct from their provider's, which is why many single-homed customers never get one.
The original field was 16 bits, giving 65,536 values, and the pool ran short in the late 2000s. RFC 6793 (which obsoleted RFC 4893) extended ASNs to 32 bits. Speakers that support it advertise the 4-octet AS capability and exchange full 4-byte paths; when a new speaker talks to an old one, it substitutes the placeholder AS 23456 (AS_TRANS) in AS_PATH and carries the real values in the optional transitive AS4_PATH attribute, which the old router passes along untouched. Seeing AS 23456 in a path or in a router's configuration is a reliable sign that a 2-byte-only device is involved somewhere. RFC 5396 defines the two text formats: asplain (AS65546) and asdot (1.10, meaning 1 x 65536 + 10). Today all RIRs issue 4-byte numbers by default, including LACNIC, whose region has many ASNs above 260000.
Several ranges are carved out. RFC 6996 reserves 64512-65534 and 4200000000-4294967294 for private use, the BGP equivalent of RFC 1918; they are common between a single-homed customer and its provider, or inside large data-center fabrics where every rack gets its own AS. RFC 5398 sets aside 64496-64511 and 65536-65551 for documentation. AS 0 is reserved and RFC 7607 says it must not be used in a path, although an AS0 ROA is a legitimate RPKI statement meaning that a prefix should not be routed. RFC 7300 reserves the last number of each space, 65535 and 4294967295.
One prefix normally has one origin, but multiple-origin AS (MOAS) situations are legal: anycast services, DDoS scrubbing providers that announce a customer's prefix during an attack, or transitions between providers. They are also what an origin hijack looks like, so an unexpected second origin deserves an RPKI check before any conclusion. Finally, IP-to-ASN mapping reflects routing, not ownership: the same address can be registered to one company and originated by another.
Common Errors and How to Fix Them
- ProblemA private ASN from a customer session leaks into paths sent to transit providers.
- FixStrip private ASNs on eBGP sessions toward upstreams (remove-private-as on Cisco IOS, remove-private on Junos) and have upstreams reject paths containing private or reserved numbers.
- ProblemAn older router shows AS 23456 for a peer and the session or policy does not behave as expected.
- FixUpgrade or enable 4-octet ASN support so the capability is negotiated, and write policies against the real 4-byte number; AS 23456 must never be used as a configured ASN.
- ProblemEntering an asdot value like 1.10 into a field that expects asplain, producing the wrong AS.
- FixConvert with high x 65536 + low (1.10 becomes 65546) and standardize on asplain in documentation, tickets and IRR objects.
- ProblemTreating the ASN's registered country as the location of the traffic source.
- FixUse the ASN only to identify the operator. Large networks span continents; for location use latency or the operator's published geofeed.
- ProblemConcluding a hijack because an IP maps to two different origin ASNs.
- FixCheck for anycast or a DDoS mitigation provider first, then validate each origin against RPKI ROAs; only an unauthorized origin with no business explanation points to a hijack.
Frequently Asked Questions
What is an ASN and who needs one?
An Autonomous System Number identifies a network that runs BGP with its own routing policy. You need a public ASN when you connect to two or more upstream providers, peer at an Internet exchange, or announce your own address space independently. A business with a single ISP and provider-assigned addresses usually does not need one; if it runs BGP with that ISP, a private ASN is enough.
How do I get an ASN in Latin America?
Through LACNIC, either directly as a member or via a national registry in countries that have one, such as NIC.br in Brazil or NIC México. You must show you will be multihomed or have a unique routing policy and agree to the registry's terms. Allocation is usually quick once the organization is a member, and the number issued will normally be 4-byte.
Why does my IP show my provider's ASN instead of my company?
Because your provider originates the route. If you use addresses assigned by your ISP and do not run BGP yourself, the ISP announces the aggregate that contains your block, so lookups return its ASN. Even if you run BGP with a private ASN, the provider strips that number before sending the route upstream. Only your own public ASN would appear as origin.
What does AS 23456 mean in a BGP path?
AS 23456, called AS_TRANS, is a placeholder defined for the transition to 4-byte ASNs. A router that supports 4-byte numbers writes 23456 in the old AS_PATH whenever a real number does not fit in 16 bits, and carries the true path in AS4_PATH. If you see it, some device along the path does not understand 4-byte ASNs.
Can I use a private ASN to peer with my ISP?
Yes, it is common for single-homed customers. You run BGP with the provider using a number from 64512-65534 or the 4-byte private range, and the provider removes it before propagating your routes. It stops working once you want a second provider, since both would need to see a consistent public origin; at that point request your own ASN from your RIR.
Academic Documentation
Protocol context and primary references
REST API Documentation
v1.0GET /api/tools/asn-lookup
curl -X POST https://epcybertools.com/api/tools/asn-lookup \
-H "Content-Type: application/json" \
-d '{"ip":"8.8.8.8"}'
{
"success": true,
"results": [
{ "test": "Sample Check", "status": "pass", "message": "All clear" }
]
}
Usage Examples
# IP to origin ASN via DNS (reverse the octets: 203.0.113.5 -> 5.113.0.203)
dig +short 8.8.8.8.origin.asn.cymru.com TXT
# ASN to name and registry
dig +short AS15169.asn.cymru.com TXT