How do I prevent router from changing my ip address

[GeforceUser]

I like to play a lot of games with my friends online. For this I had opened a port on my computer so that they can connect my computer and join the game online. Last time when I was playing, due to some power failure the system restarted and because of that we have to redo everything. But my friends complained that they are unable to connect. When I checked I found that my router's IP address changes everytime it starts. Why? Can I prevent this?

[Archibald]

For any servers (including for games) behind the router, it is better to assign IP addresses manually rather than automatic via DHCP in order to keep them constant (static). This is reason why your friends are unable to connect your computer.

So set your computer with a static IP address, Subnet Mask and Gateway; and Primary and Secondary DNS. Configure your port forwarding based on this settings.

That's it.

[Aleksandra]

Dial-up connections receive a new IP address each time a new connection is made. This is not recommended for gaming computers such as yours. If you are using DMZ and port forwarding then whatever computers are involved in that should really be static anyway. So configure an IP in your network properties, and turn off DHCP on your router.

[BiGMaCHiNe]

You can prevent the changing of the IP address but according to me your external IP address is assigned by your ISP. So that shouldn't be the cause of your problem. Also changing the IP address of your local network shouldn't work.