An IP alias is an extra IP address which may be in a completely
  different subnet and domain than a host's primary IP address. This is
  often desired by WWW presence providers. An IP aliasing is a step
  beyond a DNS alias, which is an extra hostname for the same IP
  address. HTTP requests do not include the host name, so a server has
  no way of knowing the host name to which a particular request was
  sent. A server *can* determine the IP address to which a particular
  request was sent and respond accordingly, so IP aliasing allows one to
  have multiple independent servers on a single machine.

  Patch 1356 to IRIX 5.3 allows IP aliasing. IRIX 6.2 and later does IP 
  aliasing without patches. The interim solution once available from
  Silicon Surf is no longer available.

  See also

      http://www.amazing.com/internet/virtual-homer.html
      http://www.apache.org/docs/virtual-host.html
      http://www.thesphere.com/~dlp/TwoServers/

  for general information and descriptions of other implementations.