What Is an AS (Autonomous System)?

What is an AS (Autonomous System)?

An Autonomous System (AS) is a network or group of IP addresses operated under a single administrative authority that shares a common routing policy. Each AS is assigned a unique number known as an Autonomous System Number (ASN), which allows it to exchange routing information with other networks on the internet. Together, ASes form the backbone of global internet routing, enabling traffic to move efficiently between different providers, organizations, and regions.

Autonomous Systems are managed by internet service providers (ISPs), data centers, enterprises, universities, or government networks. They use the Border Gateway Protocol (BGP) to advertise which IP prefixes they control and to establish optimal paths for data transmission across interconnected networks.

How Autonomous Systems Work

Each AS connects to other ASes through BGP peering sessions. These relationships define how traffic enters and exits the network. When data is sent across the internet, routers consult BGP tables built from AS advertisements to determine the best path to the destination.

For example, when a mail server at mail.example.com sends an email, packets travel through multiple ASes before reaching the recipient’s server. Each AS along the route uses its routing policy to forward the data to the next network, ensuring efficient delivery across different providers.

AS relationships generally fall into three categories:

• Transit: One AS pays another to carry its traffic to the rest of the internet.
• Peering: Two ASes exchange traffic between their customers directly, often without cost.
• Private: An internal AS used by a company or organization without public routing.

The Internet Assigned Numbers Authority (IANA) and regional internet registries such as ARIN, RIPE NCC, and APNIC are responsible for allocating ASNs worldwide.

Why Autonomous Systems Matter in Email Security

Understanding AS relationships helps email security teams trace the origin of messages and identify patterns of malicious behavior. By mapping IP addresses to their corresponding ASNs, organizations can see which networks are responsible for sending or relaying messages under their domain.

Analyzing Autonomous System data supports:

• Identifying suspicious senders or compromised networks
• Tracing spoofed or phishing campaigns back to their source
• Monitoring email traffic from unexpected geographic or network regions
• Evaluating third-party vendors’ infrastructure and trustworthiness

Because spammers and attackers often exploit poorly managed or untrusted ASes, visibility into ASN-level data helps improve DMARC reporting and deliverability analysis.

Autonomous Systems and DMARCeye

DMARCeye integrates Autonomous System data into its reporting and visualization tools. By linking IP addresses found in DMARC aggregate reports to their ASNs, DMARCeye helps organizations see exactly which networks are sending messages on their behalf, or impersonating their domain.

The platform highlights risky or unfamiliar ASNs that fail authentication or appear in unauthorized sending patterns. This intelligence allows security teams to block or investigate suspicious networks and maintain a clean, trusted sending footprint across the global email ecosystem.

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.