A router forwards each packet by looking up the destination address in its routing table. Routes get into the table in three ways: directly connected networks (from the router's own configured and active interfaces), static routes an administrator types in, and dynamic routes learned from routing protocols. A default route, 0.0.0.0/0 in IPv4 or ::/0 in IPv6, matches anything not otherwise known and is often called the gateway of last resort. If no route matches and there is no default, the router drops the packet and sends back an ICMP Destination Unreachable message.
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