Servers are computers on a network providing services to computers that can contact them. This removes duplication and hence, expense. A Unix server is called a daemon. Examples include:
File servers hold programs and data files used in common by computers.
Print servers control a shared printer and manage a print queue.
Database servers manage databases that can be accessed online. They all use Structural Query Language (SQL, often pronounced "Sequel"). Database servers include Oracle, IBM DB2 and Microsoft SQL Server.
Proxy servers acts as an intermediary between a local network server and the external Internet.
Mail server - sends /forwards E-mails using protocols that reside at level 3 (application layer) of OSI/RM:
MIME is a service that allows E-mail servers to associate different file types with different applications so that the correct application can be activated automatically to deal with different types of file attached to E-mails (e.g Photoshop to display an E-mailed image).
Mail List Servers hold and manage mailing lists.
Media servers distribute streaming audio and video. In the case of slow Internet connections, video and audio is downloaded to a buffer on the destination machine before being displayed to avoid pauses in the middle when played.
FTP servers transfer files without needing mail systems. Necessary for files of 2MB or more which would slow mail servers to a crawl. Most FTP servers allow anonymous login (no password) as otherwise passwords would have to be sent unencrypted.
Certificate server validates encryption keys (which are strings of text). Used to authenticate users before they transmit information.
Directory server identifies resources (files, E-mail address lists, other servers etc.) on a network. Also allows resources to be centralised for a network, to avoid duplication. Uses protocols X.500 or its successor Lightweight Directory Access Protocol (LDAP).
Catalog server indexes databases, files and information on a network, allowing keyword and textstring searches. Bots
Fax server provides a series of fax modems to process all outgoing and incoming faxes for a system. More than one fax modem allows faxes to be sent in parallel.
Transaction server update databases when a commercial transaction takes place. Also verifies that all parts of a transaction have taken place correctly.
Mirrored server is a server that handles a mirror site, one which duplicates data or resources (on the same or different computer/network). Provides redundancy in case one set of resources fails.
Store web pages that can be requested over the World Wide Web. Holds static web pages and those generated dynamically by Common Gateway Interface (CGI) applications on the server. Common web servers include:
Both the operating system on a computer and the web server can give different levels of access to files so that they can be read/written/executed (permissions). Web pages can either be open to anyone ("anonymous access") or to paying/registered users only (listed on an ACL, Access Control List).
Aliases: Web servers allow different URLs to be mapped to other names for convenience of typing (e.g. my web site at www.tripod.com/~RichardBowles is more commonly accessed by typing richardbowles.tripod.com). This allows the web administrator to move files around on a system as long as the alias points to the right location.
Web servers maintain logs of all requests for web pages including the date/time the request was made, the entry point to the domain (not necessarily the home page) and any errors (such as requests for non-existent web pages).
The normal performance of a network working correctly is its baseline performance (in packets per second). This is monitored and compared to peformance during periods of high/low traffic or when the network is running unusually quickly or slowly.
HTML is a client-side language - the browser on the client's computer receives the HTML code and interprets it. Server-side language run on the server only. The client only sees the output (web pages) that they generate:
Translates domain names into IP addresses. It uses the Domain Name Hierarchy, with each level being handled by its own server.
Root level domain (Root level server) Top level domain (.com, .org, .net etc. and country codes such as .uk) - Primary server Second level domain (.ac.uk, .co.uk - Secondary server) .www, .ftp
Allows access to Usenet groups using Network News Transfer Protocol. Newsgroups are subdivided in a similar manner to web addresses e.g. uk.adverts.computer. Group uk is subdivided into adverts, subdivided into computers.
Newsgroups may have a moderator - a user who decides on content and whether articles are accepted. Security can be achieved by using Secure Sockets Layer (SSL) session, which needs a digital certificate.
Newsfeed - One newsgroup server contacting another to access a newsgroup.
This allows messages to be sent to a destination computer and displayed immediately on the screen (rather like text messages on mobile phones). You log on to an IM service, and add names to a "buddy list". Whenever you log on to the service, the online/offline status of your buddies is shown. Any message sent to an online buddy is displayed immediately.
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