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In a client/server arrangement, network
services are located in a dedicated computer whose only function is to
respond to the requests of clients. The server contains the file,
print, application, security, and other services in a central computer
that is continuously available to respond to client requests. Most
network operating systems adopt the form of client/server
relationships. Typically, desktop computers function as clients and
one or more computers with additional processing power, memory, and
specialized software function as servers.
Servers are designed to handle requests from many clients simultaneously. Before a client can access the server resources, the client must identify itself and be authorized to use the resource. This is usually done by assigning each user an account name and password. A specialized authentication server acts as an entry point, guarding access to the network, and verifies this account information. By centralizing user accounts, security, and access control, server based networks simplify the work of network administration. The concentration of network resources such as files, printers, and applications on servers also makes the data they generate easier to back up and maintain. Rather than having these resources spread around on individual machines, they can be located on specialized, dedicated servers for easier access. Most client/server systems also include facilities for enhancing the network by adding new services that extend the usefulness of the network. The distribution of functions in client/server networks brings substantial advantages, but also incurs some costs. Although the aggregation of resources on server systems brings greater security, simpler access, and coordinated control, the server introduces a single point of failure into the network. Without an operational server, the network cannot function at all. Additionally, servers require a trained, expert staff to administer and maintain them, which increases the expense of running the network. Server systems require additional hardware and specialized software that adds substantially to the cost.
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