client identification on server

What is a simple way for the server to idenitfy what client a command comes from.


For example, I am writing a simple position server for a demonstartion. I need it so that when multiple clients send their positions, the server can tell where the information is coming from..

i.e.

client1 sends it position and the server puts it in a stuct..

client1.x
client1.y
client1.z

and client2 goes in

client2.x
client2.y
client2.z

etc...

That way the server can send the structs to each client so client1 can know where client2 is, etc


Thank you in advance for any help.