ASN Lookup

Our free ASN lookup tool retrieves detailed information about any Autonomous System Number. Enter an ASN (e.g., AS13335) or an IP address to find the organization name, allocated IP prefixes, RIR assignment, and registration date. This ASN lookup is useful for network engineers, security researchers, and anyone investigating IP address ownership and internet routing.

What Is an ASN?

An Autonomous System Number (ASN) is a unique identifier assigned to a network or group of networks that operate under a single administrative policy. ASNs are essential to the Border Gateway Protocol (BGP), which routes traffic between different networks on the internet. Every major ISP, cloud provider, and content delivery network has at least one ASN. The IANA Autonomous System Numbers registry maintains the authoritative list of all AS number allocations to Regional Internet Registries.

Who Gets an ASN?

ASNs are assigned by Regional Internet Registries (RIRs):

• ARIN — North America
• RIPE NCC — Europe, Middle East, and parts of Central Asia
• APNIC — Asia-Pacific
• LACNIC — Latin America and the Caribbean
• AFRINIC — Africa

Organizations that need to exchange routing information with multiple networks — such as ISPs, cloud providers, universities, and large enterprises — typically hold an ASN. Obtaining one requires justification of multi-homed connectivity.

IP Prefixes and BGP Announcements

Each ASN announces one or more IP prefixes (CIDR blocks) via BGP. These prefixes tell other networks which IP addresses are reachable through that ASN. A large ISP may announce thousands of prefixes covering millions of IP addresses, while a small organization might announce just one or two. You can use this ASN lookup tool to see all prefixes currently announced by an ASN and quickly determine the scope of a network’s address space.

How BGP Routing Works

BGP is the protocol that glues the internet together. When an ASN announces its prefixes, neighboring ASNs receive these announcements and propagate them further. Each router builds a table of reachable prefixes and the AS paths to reach them. Traffic follows the AS path with the fewest hops (or the best policy match). BGP route hijacking — where an unauthorized ASN announces someone else’s prefixes — is a real security concern that BGP monitoring services track. Running an ASN lookup on suspicious announcements is a common first step in incident response.

Datacenter vs. Residential ASNs

ASNs can be broadly categorized as datacenter/hosting (used by cloud providers, hosting companies, and CDNs) or residential/ISP (used by internet service providers serving end users). This classification is useful for fraud detection, ad verification, and understanding traffic patterns. Traffic from datacenter ASNs is more likely to be automated (bots, scrapers, VPNs), while residential ASN traffic typically comes from real users.

ASN Numbers: 16-bit vs 32-bit

Originally, ASNs were 16-bit numbers (0–65535), but as the internet grew, this space ran out. In 2007, 32-bit ASN support (0–4294967295) was introduced via RFC 4893. Modern ASNs above 65535 are written in “asplain” notation (e.g., AS394711) rather than the older “asdot” format. All major routing implementations now support 32-bit ASNs, and new assignments from RIRs are typically in the 32-bit range.

ASN Lookup Use Cases

An ASN lookup helps answer common questions about network ownership and routing:

• Who owns this IP? — Find the organization behind any IP address by looking up its ASN
• What IPs does a company control? — View all prefixes announced by a specific ASN
• Is this a hosting IP? — Determine if traffic comes from a datacenter or residential network
• Peering analysis — Understand an organization’s upstream providers and peering relationships
• Threat intelligence — Identify networks associated with abuse, spam, or attack traffic

Security teams frequently perform an ASN lookup as part of threat intelligence workflows to map malicious infrastructure back to specific hosting providers or ISPs.

IXP and Peering

Internet Exchange Points (IXPs) are physical locations where multiple ASNs connect to exchange traffic directly, rather than routing through upstream transit providers. Peering at IXPs reduces latency, lowers costs, and improves reliability. Major IXPs like AMS-IX, DE-CIX, and LINX handle terabits of traffic per second. Checking an ASN’s peering relationships reveals how well-connected a network is.

Related Tools

To convert an IP address between different formats, use our IP Address Converter. For resolving an IP to a hostname, try our Reverse DNS Lookup.