User On Internet Or Intranet?

I'd like to check if the user accessing my site is from the internet or from the intranet, based on which I display different messages in a page. I propose to check the REMOTE_ADDR server variable to determine the user.

But I'm not sure if checking if the REMOTE_ADDR = 127.x.x.x or 10.x.x.x would suffice. Thus if the first hex part of the ip address is 10 then he's from a lan, or if t's 127, it's the local machine. Otherwise he's from internet. Would this be a foolproof way of determining where the user is from? Just forgot my TCP/IP lessons .