Linux internet gateway able to route to address with no route set up

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http://serverfault.com – Synopsis I can ping and see the http config page on my cable modem at its fixed address of from computers on the local network behind my Linux internet gateway. That gateway is the default route for machines on the network. netstat does not indicate any route to 192.168.*, but the gateway must be routing packets to because I can reach the modem at that address from machines behind the gateway and from the gateway itself. How does the gateway know to send those packets to the internet interface instead of saying that the network is unreachable? Is there some way to (HowTos)