Client

A client is a piece of computer hardware or software that accesses a service made available by a server. There are three types of clients; fat clients, hybrid clients and thing clients. An overview of these can be seen in the table below. Clients usually connect to servers or one another creating either client-server networks or peer-to-peer networks.


Relies on local storage
Relies on local CPU
Fat client
Yes
Yes
Hybrid client
No
Yes
Thin client
No
No
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DNS


The Domain Name Server is a standard technology for managing the names of Web sites and other Internet domains. DNS technology allows you to type names into your Web browser like compnetworking.about.com and your computer to automatically find that address on the Internet.



Email Server

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Server


Definition: It is a combination of software and hardware that processes and delivers data to clients in a network.

Hardware in a server may vary from server to server based on the purpose of the server. Likewise different servers may have different processing powers, based on how much traffic they receive.
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__http://compnetworking.about.com/od/basicnetworkingconcepts/g/network_servers.htm__
__http://en.wikipedia.org/wiki/Server_(computing)__

Router


A router is a device that forwards data packets between computer networks. Clients connect to a router and any incoming or outgoing connections are managed by the router.
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Firewall

A system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.