single word requests - If you call somebody or they call you, what is the name for that person?

This can also be applied to client-server interactions (which is what I need it for).

For example, if you (the client) send data to the server or retrieve data from the server, what is the name of the server in that instance?

Or, if the server sends data to the client or the server receives data from the client, what is the name of the client in that instance?

Both of these names should be equivalent. Basically, I'm looking for a word (or words) for the other person/entity when the two are connected.