What are computer networks?

 A computer network is a group of computers linked to each other that enables the computer to communicate with another computer and share their resources, data, and applications. Computer networks are Used for everything from accessing the internet or printing a document to downloading an attachment from an email, networks are the backbone of business today. They can refer to a small handful of devices within a single room to millions of devices spread across the entire globe, and can be defined based on purpose and/or size. Some of the uses of computer networks are the following:

• Communicating using email, video, instant messaging, etc.
• Sharing devices such as printers, scanners, etc.
• Sharing files.
• Sharing software and operating programs on remote systems.
• Allowing network users to easily access and maintain information.

According to the communication requirements, multiple types of network connections are available. The most basic type of network classification depends on the network's geographical coverage and other types are based on operational requirements.

1. Personal Area Network (PAN) : PAN is the most basic type of computer network. This network is restrained to a single person, that is, communication between the computer devices is centered only to an individual’s work space. PAN offers a network range of 10 meters from a person to the device providing communication. Examples of PAN are USB, computer, phone, tablet, printer, PDA, etc.
2. Local Area Network (LAN) : LAN is the most frequently used network. A LAN is a computer network that connects computers together through a common communication path, contained within a limited area, that is, locally. A LAN encompasses two or more computers connected over a server. The two important technologies involved in this network are Ethernet and Wi-fi. Examples of LAN are networking in a home, school, library, laboratory, college, office, etc.
3. Wide Area Network (WAN) :WAN is a type of computer network that connects computers over a large geographical distance through a shared communication path. It is not restrained to a single location but extends over many locations. WAN can also be defined as a group of local area networks that communicate with each other. The most common example of WAN is the Internet.
4. Wireless Local Area Network (WLAN) :  WLAN is a type of computer network that acts as a local area network but makes use of wireless network technology like Wi-Fi. This network doesn’t allow devices to communicate over physical cables like in LAN but allows devices to communicate wirelessly. The most common example of WLAN is Wi-Fi.
5. Campus Area Network (CAN) : CAN is bigger than a LAN but smaller than a MAN. This is a type of computer network which is usually used in places like a school or college. This network covers a limited geographical area that is, it spreads across several buildings within the campus. Examples of CAN are networks that cover schools, colleges, buildings, etc.
6. Metropolitan Area Network (MAN) : A MAN is larger than a LAN but smaller than a WAN. This is the type of computer network that connects computers over a geographical distance through a shared communication path over a city, town or metropolitan area. Examples of MAN are networking in towns, cities, a single large city, large area within multiple buildings, etc.
7. Storage Area Network (SAN) :SAN is a type of computer network that is high speed and connects groups of storage devices to several servers. This network does not depend on LAN or WAN.. Instead, a SAN moves the storage resources from the network to its own high-powered network. A SAN provides access to block-level data storage. Examples of SAN are a network of disks accessed by a network of servers.
8. System Area Network (SAN) :A SAN is a type of computer network that connects a cluster of high-performance computers. It is a connection-oriented and high bandwidth network. A SAN is a type of LAN that handles high amounts of information in large requests. This network is useful for processing applications that require high network performance. Microsoft SQL Server 2005 uses SAN through virtual interface adapter.
9. Passive Optical Local Area Network (POLAN) : A POLAN is a type of computer network which is an alternative to a LAN. POLAN uses optical splitters to split an optical signal from a single strand of single mode optical fibre to multiple signals to distribute users and devices. In short, POLAN is a point to multipoint LAN architecture. 
10. Enterprise Private Network (EPN) : EPN is a type of computer network mostly used by businesses that want a secure connection over various locations to share computer resources. 
11. Virtual Private Network (VPN) :A VPN is a type of computer network that extends a private network across the internet and lets the user send and receive data as if they were connected to a private network even though they are not. Through a virtual point-to-point connection users can access a private network remotely. VPN protects you from malicious sources by operating as a medium that gives you a protected network connection. 
12. Home Area Network (HAN) : Many of the houses might have more than a computer. To interconnect those computers and with other peripheral devices, a network should be established similar to the local area network (LAN) within that home. Such a type of network that allows a user to interconnect multiple computers and other digital devices within the home is referred to as Home Area Network (HAN).  HAN encourages sharing of resources, files, and programs within the network. It supports both wired and wireless communication. 
13. Internetwork: An internetwork is defined as two or more computer network LANs or WAN or computer network segments are connected using devices, and they are configured by a local addressing scheme. This process is known as internetworking. An interconnection between public, private, commercial, industrial, or government computer networks can also be defined as internetworking. Types Of Internetwork:
• Extranet: An extranet is a communication network based on the internet protocol such as Transmission Control protocol and internet protocol. It is used for information sharing. The access to the extranet is restricted to only those users who have login credentials. An extranet is the lowest level of internetworking. It can be categorized as MAN, WAN or other computer networks. An extranet cannot have a single LAN, atleast it must have one connection to the external network.
• Intranet: An intranet is a private network based on the internet protocol such as Transmission Control protocol and internet protocol. An intranet belongs to an organization which is only accessible by the organization's employee or members. The main aim of the intranet is to share the information and resources among the organization employees. An intranet provides the facility to work in groups and for teleconferences.