What Is an ASN (Autonomous System Number)?
An ASN is a unique number assigned to a network or group of networks that operates under a single routing policy on the internet. It identifies who controls a block of IP addresses and how traffic is routed between networks.
How ASNs Work
Every large network on the internet (ISPs, cloud providers, hosting companies, large enterprises) is assigned an Autonomous System Number. These networks use BGP (Border Gateway Protocol) to announce which IP address ranges they control to neighboring networks. Routers across the internet use these BGP announcements to determine the best path for traffic to reach its destination.
ASN numbers come in two formats: 16-bit ASNs (ranging from 1 to 65,535) and 32-bit ASNs (up to 4,294,967,295), introduced as the original pool was exhausted. They are typically written with the prefix AS, for example AS15169 (Google) or AS13335 (Cloudflare).
Relevance to Email and DNS
ASN data is critical for email security and deliverability. Spam filters and blocklist operators track sending reputation at the network level. If a significant number of IPs within the same ASN are flagged for spam, the entire AS can be penalized. Understanding which ASN your mail server belongs to helps you assess your sending reputation and diagnose delivery issues.
ASN lookups are also essential for investigating suspicious email sources, identifying the hosting provider behind an IP address, and verifying that your DNS infrastructure is hosted on reputable networks.
Frequently Asked Questions
How do I find the ASN for an IP address?
You can use an ASN lookup tool to query public WHOIS databases and routing tables. The result shows the AS number, the owning organization, and the IP prefixes they announce.
Why does ASN matter for email?
Spam filters and blocklists track reputation at the ASN level. If many IPs within the same ASN send spam, the entire network's reputation can suffer, affecting deliverability for all senders on that network.
Who assigns ASN numbers?
ASNs are assigned by Regional Internet Registries (RIRs) such as ARIN, RIPE NCC, APNIC, LACNIC, and AFRINIC, coordinated by IANA which manages the global ASN pool.