Can two applications listen to the same port?

Question

Can two applications on the same machine bind to the same port and IP address? Taking it a step further, can one app listen to requests coming from a certain IP and the othe

Answer 1

Short answer:

Going by the answer given here. You can have two applications listening on the same IP address, and port number, so long one of the port is a UDP port, while other is a TCP port.

Explanation:

The concept of port is relevant on the transport layer of the TCP/IP stack, thus as long as you are using different transport layer protocols of the stack, you can have multiple processes listening on the same : combination.

One doubt that people have is if two applications are running on the same : combination, how will a client running on a remote machine distinguish between the two? If you look at the IP layer packet header (https://en.wikipedia.org/wiki/IPv4#Header), you will see that bits 72 to 79 are used for defining protocol, this is how the distinction can be made.

If however you want to have two applications on same TCP : combination, then the answer is no (An interesting exercise will be launch two VMs, give them same IP address, but different MAC addresses, and see what happens - you will notice that some times VM1 will get packets, and other times VM2 will get packets - depending on ARP cache refresh).

I feel that by making two applications run on the same : you want to achieve some kind of load balancing. For this you can run the applications on different ports, and write IP table rules to bifurcate the traffic between them.

Also see @user6169806's answer.