Unix machines connected by Ethernet usually speak to each other using a set of protocols known collectively as TCP/IP. Each machine has a name, but is also known to the rest of the network by its Internet Protocol address, or IP number. Workstations are typically identified by a decimal number consisting of 4 fields, each containing an integer in the range 1-255.
For example, at OUCS there is a Unix host whose Internet name is linux.ox.ac.uk.ox.ac.uk and whose IP number is 163.1.2.4
In the following sections, the IP number can usually be substituted for
the machine name. The equivalence of host name and IP number is established
in a file called /etc/hosts for important local machines. All
others are looked up in a distributed database across the Internet called
the Domain Name Server (DNS).