[U2Lynne]

What the instructions need to say is you need a static *local* IP. That is the one that looks like 192.168.1.xxx.

Go to your router and turn DHCP off (or some routers allow you to have it DHCP with manual, but you need to know the range it allows for manual ones - look in your manual for that). Then go to System Preferences > Network > Built-in Ethernet > (Configure) > Configure IPv4 set to Manually; IP Address set to 192.168.1.xxx (pick your own xxx, but remember them); Router set to 192.168.1.1; the rest should be from your ISP > Apply. Now go to your router to forward your ports, the IP you want to forward them to is the one you just input - 192.168.1.xxx