This is the internet name of the remote client. If the name of the client can not be determined this variable usually holds the ip number of the client. To have an ip name in this variable depends on the client and also on the configuration of the web server. The http request does not include the name of the client. It only holds the ip number, and the web server should issue a reverse lookup request toward the DNS server to determine the ip name of the client. This is used sometimes for security reasons disallowing all clients that have no ip name. On the other hand this is a slow process and may severely impact the performance of the web server. Usually this reverse lookup is switched off.