BGP announcement
A BGP announcement is a message, defined by the Border Gateway Protocol in RFC 4271, that one network operator's router sends to its neighbors declaring it can route traffic to a given block of IP addresses, identified by its ASN. Every other network on the internet that wants to reach that address block learns the path to it only because some chain of BGP announcements, hop by hop, advertised a route there -- there's no central map of the internet's routing, just the accumulated, mutually trusted announcements every network makes to the ones it connects to. BGP trusts these announcements by default, with no built-in cryptographic proof that an announcing network actually controls the addresses it claims, which is what makes BGP hijacking possible: a network can announce a block of IP addresses it doesn't operate, and if the announcement propagates, some portion of the internet's traffic for those addresses gets misdirected to it instead of the legitimate operator.
By AdminKit team, Netbase JSC · Updated