IP addresses
To allow communication over heterogeneous networks, each with its own addressing standard, every location in a network needs a globally unique IP address. A computer that is connected to the Internet needs at least one IP address; a node that interconnects two networks needs two.
IP addresses are 32-bit numbers that are commonly represented as a series of four decimal numbers between 0 and 255, separated by a period. An example IP address is 134.148.250.28. Some IP addresses have special meanings; for example, the IP addresses 127.0.0.0 and 127.0.0.1 are reserved for loopback testing on a host. If a connection is to be made from a client to server, both running on the same machine, the address 127.0.0.1 can be used. This address loops back to 127.0.0.0, the localhost. The address 0.0.0.0 is used by IP to identify the default route out of a node.
A system's network file contains the links between network devices and IP addresses. The IP network information can usually be found in the file /etc/networks on a Linux system.