For an Intranet project, seems rather strange that you want to associate users with a specific IP range. What if that changes? Don't you have any other characteristics you can use?
How about authenticating the users? Single Signon using perhaps LDAP (which is available as a plugin), why not use the build in ACL system.
If they are truly guests with no name, no face, I can imagine the following architecture:
create a guest account with basically no permissions.
authenticate any guest in the background via an authentication plugin. In the authentication plugin you can check the IP.
create a view level / access group for that guest user and use the ACL to assign menu items and content.
Even if you don't authenticate users, I would still look into a solution based on the Joomla ACL.