If you want the servers ip - just use wire-shark
if you want a clients(players) ip then you need to backdoor it-
*you need access to the servers files to put in the backdoor
*you need a way to access the backdoor ( i like odium's but i forget the name)
*Admins can find it if they look through the files (Obv)
*just google anything else you don't know its not that difficult