You are asking to set up a basic forwarding rule for a firewall.
You can use a hardware firewall to do this, or you can use iptables.
So, if your setup is something like: Cable modem -> Iptables firewall -> switch -> your computer
Your iptables firewall would need something like:
Code:
iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to 192.168.1.50:80
iptables -A FORWARD -d 192.168.1.50 -p tcp --dport 80 -j ACCEPT
to forward port 80 requests to the firewall to your internal computer instead.