you have to foward all the ports that the mir server uses
do the following:
1. install mir server
2. foward all appropiate ports
3. set up DMZ host on router to your IP computers network up
(to find out your computers ip click start/run type in CMD then type in ipconfig)
4. download a server launcher.. look about on forums there are some great ones going
use it to change the IP address in all the files to your normal internet IP address not your computer network IP.
5. start server after saving all files. (Bare in mind only outside of the router players can get in) - (for yourself and other people to get in you need to add a route)
to add a route go into command prompt again (click start/run type CMD) then type in the following
route ADD xxx.xxx.xxx.xxx MASK xxx.xxx.xxx.255 xxx.xxx.xxx.xxx METRIC 1
the first set of x's = your internet IP address the set of x's after MASK are the first 3 combination of numbers also from your internet IP address.. the last set of x's is your computers IP address
if you need more help i am willing to write a full guide with images later on and will post on forums
(i just want to say thanks to thedeath for the adding routes malarky

)