You can do this. This would simply mean installing a network switch between your router and your ISP. You would need seperate public IPs for each device connected to this network switch. For example assume that you have a cable modem which has a router connected to it. If you wanted to connect a server directly to the public internet you would disconnect the router from the cable modem, and put a network switch between the two. Then connect the computer to the network switch. For the computer to work you would need to have two public IP addresses. One for the router and one for the computer. If you only have a single public IP then you can not have two devices.