There are two ways to reference an Internet host: an alphabetic name and a series of numbers. The alphabetic version is called the ``host name`` and the numeric the ``IP address''. At CERN, all host names end with 'cern.ch', and this suffix is called the ``domain name''. Since hosts may change their addresses, it is good practice to always use the host name. Normally if your system is configured correctly it should refer to the central CERN name servers and you should always be able to use host names. However, if, for some reason, the IP address cannot be found in the nameservers, you can try the IP address directly if you happen to know it.