BGP Routing Basics
BGP (Border Gateway Protocol) is how the world's ~75,000 autonomous systems agree on where traffic should go. There is no central map of the Internet; instead, each network announces the IP prefixes it can deliver, and these announcements ripple out across the globe.
Announcements and paths
When AS64500 announces 203.0.113.0/24, its neighbours learn "to reach that block, come through AS64500." Each network that re-announces it prepends its own ASN, building an AS path such as AS64999 AS64998 AS64500. Routers prefer the most specific prefix, then the shortest/most-preferred path.
Key ideas
| Term | Meaning |
|---|---|
| Prefix | A block of IPs being announced (e.g. a /24) |
| Origin AS | The AS that first announces the prefix |
| AS path | The chain of ASes a route traversed |
| Peering / transit | Whether networks exchange routes as equals or one pays the other |
Why hijacks happen
BGP trusts what it is told. If a network announces a prefix it does not own — by mistake or maliciously — and its announcement is more specific or shorter-path, traffic can be diverted. Famous outages have come from exactly this. The mitigation is RPKI route origin validation, which lets networks reject announcements from unauthorized origins — see RPKI and ROA explained.
Find the origin AS and prefix for any IP with our ASN lookup, then validate it with the RPKI/ROA checker. Background: what is an ASN.
Note: BGP optimizes for reachability and policy, not security or shortest geographic path. That pragmatism is why the Internet scales — and why origin validation matters.