Null routing (also called blackholing) is a defensive control, the blunt-instrument response a DDoS test often reveals as the only fallback when finer mitigation is missing. It installs a route that sends all traffic for a targeted IP to a null interface, where the kernel silently discards it. The flood stops reaching the network, but so does every legitimate packet to that address: the host is sacrificed to protect the rest of the infrastructure. RTBH (remotely triggered black hole) automates this across a network via BGP.
Why it matters in DDoS testing
Null routing is a containment tactic, not protection: it completes the attacker's goal of making the target unreachable. A test exposes how much of the service depends on a single IP, so operators know the real cost of pulling the trigger, and pushes the defense upstream toward scrubbing that drops attack traffic while keeping the host online.
For how to measure resilience instead of relying on blackholing, see DDoS Resilience Testing.