You keep seeing BGP mentioned next to ASNs and routing, and how they fit together is not obvious. BGP, the Border Gateway Protocol, is the system that lets independent networks share routes across the internet, and it uses ASNs to do it. Here is what BGP is, how it works, and why every ASN depends on it.
What is BGP (Border Gateway Protocol)?
BGP, or Border Gateway Protocol, is the routing protocol that connects the internet's independent networks. It lets each autonomous system announce which IP ranges it can reach, so routers everywhere can work out a path from one network to the next. BGP is what turns thousands of separate networks into one reachable internet.
Every network that runs BGP is identified by an Autonomous System Number (ASN). When you load a website, your traffic crosses several autonomous systems, and BGP is the protocol that chose the route it took.
How does BGP work?
BGP works by having neighboring networks, called peers, announce routes to each other. Each network tells its peers which IP prefixes it can reach and through which path. Routers collect those announcements into a routing table, pick the best path to every destination, then pass the information on to their own peers.
Each route carries an AS_PATH, the ordered list of ASNs the announcement has passed through. BGP uses the AS_PATH and other attributes to choose the best route and to prevent loops. The whole system runs on trust between networks, so a wrong or malicious announcement can reroute traffic, which is why routing security matters.
How BGP uses ASNs
ASNs are the identifiers BGP routes between. Every BGP announcement is tagged with the ASN that originated it, and the AS_PATH records every ASN along the way. Without ASNs, BGP would have no consistent way to name who announces a route or to detect loops. For the full definition, see what an ASN is.
To see which ASN sits behind any IP address, use the ASN Lookup tool.
eBGP vs iBGP
BGP runs in two modes. External BGP (eBGP) exchanges routes between different autonomous systems, which is how networks connect across the public internet. Internal BGP (iBGP) shares those routes inside a single autonomous system so every router in the network agrees on how to reach the outside world.
Why BGP matters for security and fraud
Because BGP relies on networks trusting each other's announcements, it can be abused. In BGP hijacking, a network announces IP ranges it does not own, pulling traffic toward itself. Route leaks can do similar damage by accident. The ASN that originates a route is the first thing analysts check when a hijack is suspected.
The ASN behind an IP is also a practical risk signal. Traffic from a hosting or data-center ASN is more likely to be automated than a residential connection. Abstract's IP Intelligence API returns the ASN and its type, plus flags such as is_hosting, is_vpn, and is_proxy, so you can score requests in real time.
How to see the ASN behind a route
To find the ASN that announces a given IP address, enter the IP into an ASN lookup tool. You get the ASN, the operator's name, and its type. Look up any ASN with Abstract's free tool, or read how IP-to-ASN mapping works under the hood.
Frequently Asked Questions
What does BGP stand for?
BGP stands for Border Gateway Protocol. It is the routing protocol that lets the internet's autonomous systems exchange reachability information and choose paths between networks.
What is BGP used for?
BGP is used to route traffic between independent networks on the internet. Each network announces the IP ranges it can reach, and BGP builds the global routing table that every router uses to forward traffic.
How does BGP use ASNs?
Every BGP route is tagged with the ASN that originated it, and the AS_PATH lists each ASN the route passes through. ASNs let BGP identify networks and prevent routing loops.
What is the difference between eBGP and iBGP?
eBGP (external BGP) exchanges routes between different autonomous systems, while iBGP (internal BGP) distributes those routes among the routers inside a single autonomous system.
What is BGP hijacking?
BGP hijacking is when a network announces IP prefixes it does not own, causing traffic to be misrouted toward it. The originating ASN is the key signal used to detect and trace hijacks.
Is BGP still used today?
Yes. BGP runs internet routing between networks, and every autonomous system on the public internet relies on it to reach and be reached.
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