A host is a computer or device that supports the TCP/IP protocol. Every host is identified by a 32-bit number called the IP address. For convenience, the 32-bit IP address number is usually expressed in a dot notation, e.g.134.121.64.1, in which the individual bytes are separated by dots. A host is also known by a host name, e.g. dns1.eecs.wsu.edu. In practice, applications usually use host names rather than IP addresses. Host name and IP address are equivalent in the sense that, given one, we can find the other by DNS (Domain Name System) (RFC 134 1987; RFC 1035 1987) servers, which translate IP address into host name and vice versa.
An IP address is divided into two parts: a NetworkID field and a HostID field. Depending on the division, IP addresses are classified into classes A to E. For example, a class B IP address is divided into a 16-bit NetworkID, in which the leading 2 bits are 10, followed by a 16-bit HostID field. Data packets intended for an IP address are first sent to a router with the same networkID. The router will forward the packets to a specific host in that network by HostID. Every host has a local host name localhost with a default IP address 127.0.0.1. The Link layer of localhost is a loop-back virtual device, which routes every data packet back to the same localhost. This special feature allows us to run TCP/IP applications on the same computer without actually connecting to the Internet.
Source: Wang K.C. (2018), Systems Programming in Unix/Linux, Springer; 1st ed. 2018 edition.