Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

In the TCP/IP networking environment, a host is basically a node on the network that has an IP address. In contrast, a router is a device that interconnects networks.

In a network environment in which multiple LANs are connected together with a series of routers, a host is often referred to as the end system or ES. For example, if the accounting department is connected to the sales department with a router, then workstations in each department are referred to as "hosts" (or "end systems"), and the router is referred to as an intermediate system. There may be a number of intermediate systems that a communication message has to cross between one end system and another.

In the IBM environment, "host" is the term normally applied to mainframe computer systems. More appropriately, they are called the "host processors." These hosts include the IBM model 3090, IBM model 4381, or IBM model 9370. These mainframes usually run the MVS (Multiple Virtual Storage) operating system, running as either XA (Extended Architecture) or ESA (Enterprise Systems Architecture). MVS is part of IBM's SAA (Systems Application Architecture). Refer to "Mainframe" for more details. Also see "SNA (Systems Network Architecture)" for related information.

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