Kind of sounds like a firewall, doesn't it?  )  Well the best security IS multi-layered....why should DNN's be any different?! 

The US Federal Reserve system has even admonished (and now required) multi-factor authentication (Usernames & Passwords are simply not enough for real security these days)!

How many of us run dedicated intranet or extranet portals?  A single portal should be available only to users from a particular client company.  So we can use either RDNS lookups, IP segment lists, or both to confirm the user is allowed to access the chosen portal from the given network...

Business case (real example):  We have both Coke and Pepsi as current clients.  Obviously both have legal departments large enough to destroy a thousand businesses of our size if their data gets into their competitor's hands...

Even if some staff member left one company for the other (forget the non-disclosures), some auditor services both, someone installs a keylogger, etc....a portal URL, Username, & Password should be useless for anywhere in the world but the proper client's network.