Let's clarify things:
1. Your public IP address is provided by your ISP.
2. If you have a static public IP address, you are paying more for it and your ISP has provided it to you (along with the subnet mask, default gateway, and DNS addreses). You must configure your router's WAN port with these addresses.
3. If you have a dynamic public IP address, it will change, usually whenever the router is restarted. You must configure your router WAN port for dynamic IP in this case. The IP changes as said above but you can determine it by going to www.whatismyip.com and it will tell you what it is.
4. There are non-routable IP addresses which are NOT seen on the internet but on the private side of a router. The 192.168.x.y format (where both x can range from 0 to 255 and y can range from 1 - 255) is not routable. This is NEVER your public IP address.
5. You do not open ports in a modem.
6. You open ports in routers when there is a reason to do so. This is done in the router configuration. You need to know whether it is TCP or UDP and you need to know the LAN IP address of the pc (or other LAN device) to which you wish traffic to be directed. This is a LAN IP address and may be in the 192.168.x.y format described above if your router's LAN IP subnet is in the format 192.168.x.0/24 or similar.
7. Since most LAN devices obtain their IP address from a DHCP server, like a router, they are subject to change. If you open a port in a router you must assign it to a LAN device that has a STATIC LAN IP. To do this you must first limit the Router's LAN DHCP range so you have room for static IP addresses and then assign a static IP address on the LAN device by entering the desired LAN IP address, subnet mask, default gateway, and dns server. Usually the default gateway and dns server is the LAN IP of the router in small netoworks. You assign LAN IP addresses on a Windows pc by right clicking on the LAN settings, selecting properties, highlighting Internet Connections (TCP/IP), and manually entering the addresses on the pop up window.
8. Review your router's manual on opening ports - it will describe the router's step by step process for doing this.
9. Whenever you access a wireless router for any matter whatsoever, connect to it using a wired connection, not a wireless connection.
10. When you assign a static LAN IP on your pc, be sure to select the proper LAN connection. If you wish to provide a static IP to your wireless port, be sure to select it, not the wired port for the static IP.
11. You can determine your private LAN IP address by opening command prompt (start, run - type in cmd and hit enter) and type at the prompt "ipconfig /all" and hit enter.