If a machine sends a UDP packet to some destination over the internet, and that destination sends a packet to the source address in a protocol that is not UDP, will the router forward that packet to the machine or drop it?
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Ports are per protocol. TCP port 9876 is not the same port as UDP port 9876. Some layer-4 protocols do not use port numbers.– Ron Maupin ♦Sep 23, 2017 at 18:51
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1 Answer
A "router" always forwards a packet when it can, ie. has a routing entry to apply. It doesn't care about the L4 protocol used nor its state.
It depends on what your understanding of "router" is. A simple router just forwards but with firewall rules, NAT and other functions applied, forwarding may or may not occur.
