domain name cannot be viewed on my local network

Hi there Im new to OS X Server, and am using 10.3.9. Yesterday i setup two websites with different domain names – these can be viewed from the internet and approriately serve the right content. My issue is that locally i cant view them. When i enter the domain name in the browser, i get a message from my router asking me for the username and password, rather than seeing the websites content. Ive set these hosts up using OS X Server Administrator, and have minimal experience with Terminal. Does someone know the answer to this. Probably an obvious or common problem for those to OS X Server? regards Sam