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?

  • 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
    Commented Sep 23, 2017 at 18:51

1 Answer 1


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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.