AS numbers show up everywhere in network troubleshooting: in MTR and traceroute output, in BGP looking glasses, in abuse reports, in firewall logs enriched with routing data. Each one stands for a network operator, but the number alone does not tell you who that is. When a path suddenly crosses an unfamiliar network, or a log is full of traffic from "AS64500-something", you need the organization behind it.
The NetTools.tech ASN Lookup answers that from the authoritative source. It finds the regional internet registry responsible for the number and reads its public registration record: holder organization, network name, registry handle, allocation block, status and key dates.
What is an ASN?
In simple terms
The internet is a network of networks. Each independently run network — an ISP, a cloud provider, a university, a large company — gets an Autonomous System Number so the others can recognize it when they exchange routes. An ASN lookup is like looking up a company registration number: you enter the number and get the registered holder.
Technical background
An autonomous system (AS) is a group of IP prefixes under one administrative control with a common routing policy. Networks announce their prefixes to each other with the Border Gateway Protocol (BGP, RFC 4271). Each announcement carries an AS path, the list of AS numbers it has crossed, which BGP uses for loop prevention and path selection.
AS numbers were originally 16-bit (0–65535). RFC 6793 introduced 32-bit numbers up to 4,294,967,295, with AS 23456 (AS_TRANS) as a placeholder for older routers. 32-bit numbers can be written in asplain (65546) or asdot (1.10) notation.
Several ranges are never assigned to organizations:
| Range | Purpose |
|---|---|
| 0 | Reserved (RFC 7607) |
| 23456 | AS_TRANS (RFC 6793) |
| 64496–64511, 65536–65551 | Documentation and examples (RFC 5398) |
| 64512–65534, 4200000000–4294967294 | Private use (RFC 6996) |
| 65535, 4294967295 | Reserved (RFC 7300) |
| 65552–131071 | Reserved by IANA |
IANA delegates blocks of AS numbers to the five regional internet registries (RIRs): AFRINIC, APNIC, ARIN, LACNIC and RIPE NCC, which assign them to network operators. Registration data is published through RDAP, the Registration Data Access Protocol (RFC 9082 for queries, RFC 9083 for the JSON format). An AS registration is an autnum object. IANA publishes a bootstrap file mapping AS number ranges to the RDAP server of the responsible RIR (RFC 9224).
Why use an ASN lookup?
- Identify networks in a path. Find out which operator runs the hop where latency or loss begins in MTR or Traceroute.
- Investigate traffic sources. Turn ASNs in logs or threat intelligence into organizations before blocking or reporting.
- Check BGP-related information. Confirm who holds an AS that appears in a route or peering request.
- Validate documentation. Recognize private, documentation and reserved numbers that should never appear on the public internet.
Who should use it?
- Network engineers working with BGP, peering and transit.
- Security analysts enriching logs and incident data.
- System administrators who need to know which provider a remote network belongs to.
- Abuse and NOC teams looking for the organization responsible for a network.
How ASN lookup works
- Parsing. The input accepts
13335,AS13335,as 13335and asdot notation such as1.10. In asdot, both parts must be 0–65535. Values above 4,294,967,295 are rejected. - Special-use check. If the number is in one of the ranges above, the page explains its purpose immediately and no request is sent. The server applies the same check.
- Registry discovery. The NetTools server loads the IANA RDAP bootstrap registry for AS numbers (cached for 24 hours) and picks the HTTPS RDAP server whose range contains the number. Queries only go to servers listed by IANA, never to user-supplied hosts.
- RDAP query. The server requests the
autnum/<number>record over HTTPS. Redirects are followed at most three times and only to HTTPS host names; responses are limited in size and the operation times out after 15 seconds. - Extraction. From the record the server takes the handle, name, country, allocation range, status values, registration and last-changed dates, up to five remark lines, and the holder: the registrant entity of kind organization, or another non-individual registrant. Individual people's names are never returned.
- Errors. A 404 from the registry means the number is not registered. If no registry lists the number as allocated, the tool says so. Registry rate limiting is reported as such.
Lookups are subject to the normal per-client rate limit. The query to the registry comes from the NetTools server in Latvia, not from your browser.
How to use the NetTools.tech ASN lookup
- Open ASN Lookup.
- Enter an AS number in any supported format, for example
AS13335,15169or1.10. You can also click one of the example chips, or open/asn?asn=AS13335directly. - Select Look up.
- Read the holder and registration details. Expand Registry remarks if present.
From the MTR report, the ASN shown for each hop links straight to this tool.
Understanding the results
| Result | Meaning | What to look for |
|---|---|---|
| AS label | The number in canonical AS<number> form | Confirms how asdot input was converted |
| Holder (heading) | Registrant organization; falls back to the network name, or "Holder not published" | The company or institution that holds the number |
| Country badge | Country code from the registry record, when published | Registration country, not where the network operates |
| Organization | Shown as "Not published by the registry" when no organization entity exists | Some registries publish only a network name |
| Network name | Short name of the AS, e.g. a provider's network label | Often matches names seen in BGP tools |
| Registry handle | Identifier of the record at the RIR | Useful for referencing the record with the registry |
| Allocation block | The range the number belongs to, when the record covers more than one number | Shows block allocations |
| Status | RDAP status values such as active | Inactive or unusual statuses deserve a closer look |
| Registered / Updated / Expires | Event dates from the record | Very recent registration of an AS seen in suspicious traffic |
| Registry remarks | Up to five description lines from the record | Operators sometimes publish NOC or policy notes here |
| Source | The RDAP server host that answered | Tells you which RIR is responsible |
Example: a hop in an MTR report
An MTR report shows a latency step at a hop whose network column reads AS64500. Selecting the ASN opens the lookup. Because 64500 is a documentation number, the real tool answers immediately without a registry query:
AS64500 has no owner. Documentation and examples (RFC 5398). Numbers in this range are never registered to an organization.
For a registered number, the result looks like this. The values are illustrative:
- AS64501 · Example Networks Ltd · badge
LV - Network name:
EXAMPLE-NET - Registry handle:
AS64501 - Status:
active - Registered: 2009-03-12 · Updated: 2024-11-02
- Source:
rdap.example.net (RDAP)
From this you know which organization operates the segment where latency increased and which registry holds its record. To reach that operator's NOC or abuse team, combine it with the contacts shown by IP Lookup or WHOIS for one of its IP addresses.
Common use cases
- Identifying the transit provider or IX participant at a hop in a traceroute or MTR.
- Looking up the network behind an IP address after finding its ASN with IP Lookup.
- Checking the registration date and holder of an AS that appears in suspicious traffic.
- Recognizing private ASNs leaking into public data, such as AS65001 in a BGP path.
- Converting asdot notation to asplain while checking router configurations.
Troubleshooting
"is not registered (the registry has no record for it)"
The responsible registry has no autnum record. The number may be returned to the pool, never assigned, or mistyped. Check whether you copied it from a reliable source.
"No regional internet registry lists this AS number as allocated"
The IANA bootstrap file does not map the number to any RIR, typically because the range is unallocated.
"has no owner" for a number from a BGP path
Private-use ASNs (64512–65534 and 4200000000–4294967294) are used inside networks and between customers and providers. They should be stripped before routes reach the global table; seeing one there suggests a configuration leak.
"The registry is rate limiting requests"
The RIR's RDAP service is throttling. Try again later.
Holder shows only a network name
Some records list the organization in a way the tool does not classify as an organization, or the registry publishes only network data. The network name and handle still identify the record.
"not an AS number"
The input contains extra characters. Use only digits, an optional AS prefix or asdot notation like 1.10.
Common mistakes
- Treating the country as the network's location. It is the registration country; networks often operate worldwide.
- Expecting announced prefixes. This tool reads registration data, not BGP routing tables.
- Confusing the AS holder with the end user of an IP. A hosting provider's ASN covers many customers.
- Looking up private ASNs. They are meaningful only inside the network that uses them.
- Mixing up asdot and decimal.
1.10is 65546, not 110.
Limitations
- Registration data only: no prefixes, BGP neighbors, routing visibility or traffic data.
- The record is only as accurate as the registry and the holder keep it; names, countries and remarks can be outdated.
- Individual contact names are deliberately not shown; for abuse contacts, use the IP-level lookups.
- Availability depends on the RIR RDAP services and the IANA bootstrap file.
- A registered ASN may not be in active use on the internet; the record does not show whether it is announced.
Security & privacy
- What is sent: only the AS number. The NetTools server queries IANA's bootstrap registry and the responsible RIR's RDAP server over HTTPS.
- No user-supplied hosts: requests go only to RDAP servers listed by IANA.
- Personal data: names of individual contacts in registry records are not displayed.
- Limits: rate limited, time limited and response-size limited.
- What not to enter: nothing sensitive is needed or accepted beyond the number.
Related tools
- IP Lookup — finds the AS that originates an IP address, plus registry and abuse data. Guide: IP lookup guide.
- WHOIS Lookup — registration data for domains and IP addresses. Guide: WHOIS guide.
- MTR — shows the ASN of every hop on a path, linked to this lookup. Guide: MTR guide.
- Traceroute — lists the hops whose networks you may want to identify. Guide: Traceroute guide.
- What Is My IP — your own public address and network.
- Category overviews: Network diagnostics guide, IP and network tools guide.
Summary
An ASN identifies a network with its own routing policy, and its registration record names the organization that holds it. The NetTools.tech ASN lookup finds the responsible registry through the IANA RDAP bootstrap, reads the autnum record over HTTPS and shows holder, network name, handle, allocation, status and dates, while explaining private, documentation and reserved numbers without a lookup. It describes registration, not routing, so combine it with IP Lookup and MTR when you investigate a path.
Frequently asked questions
What is an ASN?
An Autonomous System Number identifies a network that has its own routing policy on the internet, typically an ISP, cloud provider, content network or large enterprise. Networks exchange routes with each other using BGP, and every route carries the AS numbers it has passed through.
Why does my ASN show no owner?
Numbers in the private-use ranges 64512–65534 and 4200000000–4294967294, the documentation ranges and the reserved values are never registered to anyone, so the tool explains them without a lookup. Numbers that are not allocated to any registry have no record at all.
What is the difference between 2-byte and 4-byte ASNs?
Original AS numbers were 16-bit, up to 65535. Since RFC 6793, 32-bit numbers up to 4,294,967,295 are assigned too; they are sometimes written in asdot notation, for example 1.10 for 65546, which the lookup also accepts.
Which data source does the ASN lookup use?
The NetTools server finds the responsible regional internet registry in the IANA RDAP bootstrap file for AS numbers and then reads that registry's public RDAP autnum record over HTTPS. The answering server is shown under the result.
Does the ASN lookup show the IP prefixes an AS announces?
No. It shows the registration record: holder, network name, handle, allocation block, status and dates. Announced prefixes and BGP neighbors come from routing data, which this tool does not query; for a single IP address, IP Lookup shows the originating AS.
Why are contact names not shown?
The lookup deliberately shows only the holder organization. Names of individual people listed as contacts in registry records are never displayed.
How do I find the ASN of an IP address?
Use IP Lookup, which shows the AS that originates the IP's route. MTR also shows the ASN for each hop on a path, with links to the ASN lookup.