Hello.
You need to go into your Router's configuration page by opening your browser and entering the address : 192.169.1.1
( or your router manufacturer's equivalent )
And then go into the advanced area, Look for "Virtual Server" or something that allows you to configure PC's on the same network.
You will need to load cmd prompt on both computers and then type " ipconfig /all " on both computers in order to get the physical IP address of each machine, Each machine will have an address such as :
192.168.1.2(preferred)
192.168.1.3(preferred)
Which is listed in the command prompt.
What you need to do, Is type the IP addresses into the virtual server on your router and assign a port range for each PC as shown below.
http://i.imgur.com/3ZEPxik.jpg
Hope this helps.