what is the meaning of the "abc", "def" in the URL: <http://abc.def.xyz.com> . I understand xyz is called domain. I read somewhere abc is an application. How can i configure like this and where is it made? In a Java webapp, is it made in web.xml since i have not come across anything like that so far. Or is it made in servlet container/application server. 
Any information on this is appreciated.