Android apps calling web service design logic

I has an android apps install in my galaxy tab. This apps will call a web service in the web server which locate in the office. The web service then will call to database in another server. The web server and database server is in same LAN. My web server IP is dynamic which will change every time the modem is restarted and I do not register any domain name. How I can configure my router/modem to let the apps can call the web services which the ip is not statics? Is it possible? Please help.