WhatIsaNetwork

In the abstract, a network is a collection of points or nodes joined by connections sometimes known as arcs.

In practice, many things can have a network structure.

• A number of computers which are conncted by wires. Each computer is a node and each wire between the computers is a connection or arc.
• A group of people in a town or golf club or old boys' network, where each person is a node, and each friendship or acquaintanceship between two people, counts as an arc.
• A number of web-sites where each page is a node, and each hyperlink between them is an arc.
• An economy where a number of businesses are nodes, and contracts and customer relationships between them are the arcs.
• An ecology where each node is a species, and each arc a form of dependency of one species and another. For example one species preys on another, or a species of tree provides a home for a species of bird.

Considering all these examples, we can see immedietely that although they are all networks they are different in many other ways. Some of these differences are differences in the types of network. For example in the case of the computers and the people the connections are two-way. If Bob knows Sam then Sam knows Bob. In the case of the hyperlinked pages or the companies or the species, the connections are one-way. This page links to that, or this company buys from that, or this species eats that, but not the other way round.

Sometimes then we can think of a network as having directed arcs between the nodes.

http://www.nooranch.com/synaesmedia/ec/net2.gif

Some other things we can note about the network in the second illustration. It is not fully connected. Not every node is connected to every other. But some are connected via a path that leads through several intermediete nodes.

There are two nodes out on the top-right which don't have any path leading to the to the others. They are disconnected from the main group.