An ASN (Autonomous System Number) is a unique identifier assigned to a network or group of IP addresses that operate under a single administrative entity. It allows internet routing systems to understand which networks own and control specific ranges of IP addresses, helping traffic move efficiently and securely across the internet.
Every Autonomous System (AS) represents a network managed by an organization such as an internet service provider (ISP), cloud provider, university, or large enterprise. The ASN acts as that network’s digital identity in global routing tables.
ASNs are fundamental to how data, including email, travels across the internet. When one mail server sends a message to another, routers use Border Gateway Protocol (BGP) announcements to determine the best route. Those BGP routes are grouped by ASN, which tells other systems where the traffic originates.
In the context of email authentication and DMARC reporting, ASN data helps organizations identify where mail is actually coming from. When DMARC aggregate reports are analyzed, each sending IP can be traced back to its ASN, revealing the provider or service responsible for that traffic.
This makes ASN data particularly useful for detecting unusual sending sources or isolating potential abuse patterns, such as spoofing attempts from unauthorized networks.
Knowing the ASN associated with your mail sources provides valuable insight into your email ecosystem. If you see traffic coming from an unexpected ASN, it might indicate unauthorized sending, a misconfigured mail relay, or a compromised third-party service.
Security and deliverability teams often use ASN lookups to validate legitimate infrastructure or to investigate phishing campaigns. Over time, ASN visibility builds a clearer picture of your network perimeter, helping you enforce more accurate authentication and policy controls.
DMARCeye enriches DMARC report data with ASN and network ownership information. When you review your DMARCeye dashboard, you can see which ASNs are sending mail on behalf of your domain, how those networks perform in SPF and DKIM checks, and whether any unexpected sources appear.
By mapping authentication results to ASN data, DMARCeye helps you pinpoint unauthorized senders faster and maintain a trusted email ecosystem that aligns with your domain policies.
Sign up for a free trial of DMARCeye today and secure your email domain.
To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.