BGP Prefix Lookup

To check whether a prefix is announced in BGP, enter the exact CIDR block, such as 203.0.113.0/24, and review the visible route status. The main things to read are origin ASN, AS path, prefix length, and whether the route appears from more than one collector. If the prefix is not visible, it may be filtered, too new, too specific, or not advertised at all. For production troubleshooting, never rely on a single public view. Check multiple route collectors and compare the result with your router's own BGP table.

The origin AS is the last ASN in the AS path and the network that is advertising the prefix as reachable. For example, if a route ends with AS65010, that ASN is considered the origin for that prefix from that collector's point of view. It does not always prove legal ownership of the address block. Ownership is checked through WHOIS or RDAP, while origin AS is routing behavior. In troubleshooting, match the origin AS, RIR record, and expected route policy before deciding whether the announcement is valid or suspicious.

A BGP prefix may not be visible online for several reasons. The route may still be propagating, your upstream may not have advertised it, a prefix-length filter may be blocking it, or RPKI route origin validation may mark it invalid. Sometimes the prefix is visible only in certain regions because of policy or peering differences. Start by checking your local router, then your upstream provider, then public collectors. Also confirm the mask length. Many networks filter IPv4 prefixes longer than /24 on the public internet.

More-specific prefixes are smaller routes inside a larger block. For example, 192.0.2.0/24 is more specific than 192.0.2.0/23, and 192.0.2.128/25 is more specific than the /24. In a BGP prefix lookup, search the base block and then review any child prefixes or route table entries that fall inside it. This helps when traffic follows an unexpected path, because routers prefer the longest matching prefix. If a more-specific route exists, it can override a broader summary for matching destination addresses.

To check BGP route propagation online, test the same prefix from several looking glasses or route collectors. A route that appears in one place but not another may still be propagating, may be filtered by policy, or may not be carried by every transit path. Look for consistency in the origin ASN, AS path, and prefix length. Record the time of the test, because BGP is dynamic. During a change window, compare before-and-after outputs instead of judging from a single snapshot.

AS path shows the sequence of autonomous systems a BGP route passed through before reaching the collector. If you see something like 64500 64496 65010, the final ASN is usually the origin, and the earlier ASNs are transit networks seen on that path. The exact path can differ by location because BGP is policy driven, not simply shortest-distance routing. Use AS path to spot route leaks, unexpected transit providers, or prepending. Long paths are not always wrong, but sudden changes deserve a closer look.

To verify the owner of a BGP prefix, do not stop at the origin AS alone. Cross-check the prefix in WHOIS or RDAP, confirm the RIR allocation, compare the organization name, and review RPKI ROA status when available. A route can be visible in BGP and still be unauthorized or misconfigured. For example, a customer prefix may be originated by a hosting provider under arrangement, which can be valid. The best practice is to combine routing evidence, registry evidence, and your internal documentation before taking action.