ASN lookup
Find the autonomous system, network owner, and prefixes behind any IP.
An ASN lookup maps an IP address to its autonomous system number, the identifier for the network that announces and routes that address on the internet. The tool reports the ASN along with the organization, ISP, and domain behind it, which is useful for identifying hosting providers, ISPs, or the source of suspicious traffic. Knowing the network owner helps you judge reputation and trace where requests truly originate. Since routing and ownership can shift, SJ Monitor can keep an eye on the services you run on these networks.
Frequently asked questions
What is an autonomous system number?
An ASN is a globally unique number assigned to a network that independently controls its routing policy on the internet. Every public IP belongs to exactly one announcing ASN.
Why would I look up an ASN?
It identifies the provider or organization behind an IP, which helps with reputation checks, abuse investigation, and understanding who hosts a given service.
Is the ASN the same as the ISP?
Often, but not always. A large provider may own one ASN and resell capacity to others, so the ASN owner and the ISP shown can differ — both are useful for tracing who runs a network.
How do I read an ASN number?
It's written as AS followed by the number, like AS15169 (Google). The number is globally unique and assigned by a regional internet registry such as ARIN or RIPE.
What are the prefixes an ASN announces?
Prefixes are the IP ranges (in CIDR notation) that an autonomous system advertises to the global routing table via BGP. They tell you exactly which address blocks that network controls.
Can one organization use multiple ASNs?
Yes. Large companies and ISPs often hold several ASNs for different regions, acquisitions, or routing policies, so a single brand may appear under more than one autonomous system.
More Network & IP tools tools
Want this checked automatically and around the clock? Create a free SJ Monitor account and we'll alert you the moment something changes.