Difference between REMOTE_HOST and REMOTE_ADDR

Question

What is the difference between these two variables?

REMOTE_HOST and REMOTE_ADDR.

Answer 1

From RFC-3875：

• REMOTE_ADDR：The REMOTE_ADDR variable MUST be set to the network address of the client sending the request to the server.

      REMOTE_ADDR  = hostnumber
      hostnumber   = ipv4-address | ipv6-address
      ipv4-address = 1*3digit "." 1*3digit "." 1*3digit "." 1*3digit
      ipv6-address = hexpart [ ":" ipv4-address ]
      hexpart      = hexseq | ( [ hexseq ] "::" [ hexseq ] )
      hexseq       = 1*4hex *( ":" 1*4hex )

• REMOTE_HOST：The REMOTE_HOST variable contains the fully qualified domain name of the client sending the request to the server, if available, otherwise NULL. Fully qualified domain names take the form as described in section 3.5 of RFC 1034 [17] and section 2.1 of RFC 1123 [12]. Domain names are not case sensitive.

The REMOTE_HOST variable contains the fully qualified domain name of
   the client sending the request to the server, if available, otherwise
   NULL.  Fully qualified domain names take the form as described in
   section 3.5 of RFC 1034 [17] and section 2.1 of RFC 1123 [12].
   Domain names are not case sensitive.

      REMOTE_HOST   = "" | hostname | hostnumber
      hostname      = *( domainlabel "." ) toplabel [ "." ]
      domainlabel   = alphanum [ *alphahypdigit alphanum ]
      toplabel      = alpha [ *alphahypdigit alphanum ]
      alphahypdigit = alphanum | "-"