Forwarding

Null route

Also called blackhole route, unreachable route, Null0, discard route, routing loop

A route that drops what matches it: for addresses a router announces but does not have.

Without one, a packet for an unused address inside a summary goes to the branch by the summary and back by the branch's default route, looping until its TTL runs out. ip route add blackhole 10.0.4.0/24 drops it silently; unreachable also answers with ICMP host unreachable. The real subnets are more specific and still win. Cisco calls it a route to Null0, and routing protocols install it when they summarise.

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Learn it on real machines.