By default the Jetty server that is installed with Geoserver is listening only localhost. What you need to do is to configure Jetty to listen to the IP address of your server. The place to do this is in file "jetty.xml" that is located in a directory like /geoserver-2.5/etc.
<Set name="port"><SystemProperty name="jetty.port" default="8080"/></Set>
As you can see, the host is not set which means that jetty is listening to the default one which is localhost. Add this line for example before the "port" line and your Jetty should begin to listen to IP address xx.yy.zzz.ww after restart.
<Set name="host"><SystemProperty name="jetty.host" default="xx.yy.zzz.ww"/></Set>