You need to set up your computer for it. Go to My Computer > C drive (or what ever you use) > Windows > System 32 > Drivers > etc > Then open host. Then edit as like this
(Your Internal Ip) (

)
(Your External IP) (

)
Your internal IP can be found in command prompt, Once your in command prompt type
ipconfig then enter to find your internal ip mostly something like 192.168.1.1 or something along that line.
Your external IP can be found at
If that doesn't work I'll go into more detail with you. Depends on what OS your using.