Assuming here that your using a Cisco router. After defining the inside and outside interfaces offcourse. All you have to do is use the ip nat. somthing like this:
ip nat inside source static tcp 192.168.1.100 80 18.104.22.168 80
This is a static nat for a web server. 192.168.1.100 is the web server's inside private IP. 80 is the http port. 22.214.171.124 is the web server's public IP (the one people will type in the browser). 80 again is http port.
In English, the line above tells the router: "When ever a packet comes from the outside, requesting a connection to 126.96.36.199 with port 80. Then replace the IP whith 192.168.1.100, keep the 80 port the same, and send the packet inside to 192.168.1.100"