3. You must turn off DHCP on your router.
Just saw this and thought for anyone who might be reading this for help, i totally disagree with this.
The pc's requiring portforwarding should be set with a static IP, you can in some routers say what number you want DHCP ips to have ie 100-200, so you set the static ones to something else, of if you cant specify a range for DHCP, you just use some high numbers 200+ as DHCP allocates from the lowest first, assuming it started at 2 its gonna need 199 machines attached before running into problems.
Any pc not requiring a static IP can continue to work with the DHCP service of the router, this means you only need alter configurations on the pc's involved in getting portforwarding, rather than having to adjust every pc.