will show you what you can get.
The REMOTE_HOST being empty is possible; it happens on some DNS configurations I recall.
Adobe say this: "Because the available variables depend on the server, browser, and the types of interactions between the two, not all variables are normally available, and are represented by empty strings in the debug output." in addition they say this about CGI.REMOTE_HOST: "If the server does not have this information, it sets REMOTE_ADDR and does not set REMOTE_HOST."
What you get back in the CGI scope depends on many things in the client, and the client may even withhold some information depending on local settings or security setup etc.
Also, you should be mindful that things like an IP addres are not a definitive way of determining individual users. People can access your site through a proxy server, which presents multiple users access to the internet behind a single IP address.