I have problems hosting, i can only join games, can someone tell me why is this. also I have zonealarm. and linksy router.

I had to enable my DMZ in order to host.

Its done through your router settings, like 192.168.2.1

Go to DMZ and enable it, or disable if its enabled.

If I open the ports doing port forwarding why do i have to enable my dmz

Well, they are completely different things.

DMZ is the router firewall, so if you want to host then I suggest turning it off.

How d o you turn it off i have a linksy router