Are you the server administrator? If so, then we can help you to configure the server to allow static IP, but if not, you will have to accept the fact that you have a dinamic IP address!
To avoid this kind of problem you can use a service like DynDNS (http://www.dyndns.com/)
. You register there and everything, then you install a client in your machine and every time your IP address change, the client will update the hostname for you.