How to host an intranet website inaccessible outside the LAN?

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http://unix.stackexchange.com – I am a Linux/Unix noob, so please bear with me. Here is my situation: I have a server (Debian Squeeze) on a LAN. I have installed Nginx web server on it. The server has Internet access to download and install packages. My question: Can I host a website so that when users on the LAN type: http://abcd (no .com, .net, etc.) into their browser, the website opens? Note that they also have full Internet access, so it should not go straight to Google or some other default search engine instead. At the same time, anyone outside the LAN must not be able to access it, no matter what URL they typ (HowTos)