A computer network is an interconnected group of computers. Networks may be
classified by the network layer at which they operate according to basic reference
models considered as standards in the industry, such as the four-layer Internet
Protocol Suite model. While the seven-layer Open Systems Interconnection (OSI)
reference model is better known in academia, the majority of networks use the
Internet Protocol Suite (IP).
By scale
Computer networks may be classified according to the scale: Personal area
network (PAN), Local Area Network (LAN), Campus Area Network (CAN),
Metropolitan area network (MAN), or Wide area network (WAN).As Ethernet increasingly is the standard interface for networks, these distinctions
are more important to the network administrator than the user. Network
administrators may have to tune the network, to correct delay issues and achieve
the desired Quality of Service (QoS).
By connection method
Computer networks can also be classified according to the hardware technology
that is used to connect the individual devices in the network such as Optical fiber,
Ethernet, Wireless LAN, HomePNA, or Power line communication.Ethernets use physical wiring to connect devices. Often, they employ the use of
hubs, switches, bridges, and routers.Wireless LAN technology is built to connect devices without wiring. These devices
use a radio frequency to connect.
By functional relationship (Network Architectures)
Computer networks may be classified according to the functional relationships
which exist between the elements of the network, e.g., Active Networking,
Client-server and Peer-to-peer (workgroup) architectures.
By network topology
Computer networks may be classified according to the network topology upon
which the network is based, such as Bus network, Star network, Ring network,
Mesh network, Star-bus network, Tree or Hierarchical topology network, etc.Network Topology signifies the way in which intelligent devices in the network see
their logical relations to one another. The use of the term "logical" here is
significant. That is, network topology is independent of the "physical" layout of the
network. Even if networked computers are physically placed in a linear
arrangement, if they are connected via a hub, the network has a Star topology,
rather than a Bus Topology. In this regard the visual and operational
characteristics of a network are distinct; the logical network topology is not
necessarily the same as the physical layout.
By protocol
Computer networks may be classified according to the communications protocol
that is being used on the network. See the articles on List of network protocol
stacks and List of network protocols for more information. For a development of
the foundations of protocol design see Srikant 2004 [1] and Meyn 2007 [2]
Types of networks:
Below is a list of the most common types of computer networks in order of scale.
Personal Area Network (PAN)
A personal area network (PAN) is a computer network used for communication
among computer devices close to one person. Some examples of devices that
may be used in a PAN are printers, fax machines, telephones, PDAs or scanners.
The reach of a PAN is typically within about 20-30 feet (approximately 6-9
Meters).Personal area networks may be wired with computer buses such as USB[3] and
FireWire. A wireless personal area network (WPAN) can also be made possible
with network technologies such as IrDA and Bluetooth.
Local Area Network (LAN)
A network covering a small geographic area, like a home, office, or building.
Current LANs are most likely to be based on Ethernet technology. For example, a
library will have a wired or wireless LAN for users to interconnect local devices
(e.g., printers and servers) and to connect to the internet. All of the PCs in the
library are connected by category 5 (Cat5) cable, running the IEEE 802.3 protocol
through a system of interconnection devices and eventually connect to the internet.
The cables to the servers are on Cat 5e enhanced cable, which will support IEEE
802.3 at 1 Gbit/s.The staff computers (bright green) can get to the color printer, checkout records,
and the academic network and the Internet. All user computers can get to the
Internet and the card catalog. Each workgroup can get to its local printer. Note that
the printers are not accessible from outside their workgroup.
Typical library network, in a branching tree topology and controlled access to
resourcesAll interconnected devices must understand the network layer (layer 3), because
they are handling multiple subnets (the different colors). Those inside the library,
which have only 10/100 Mbps Ethernet connections to the user device and a
Gigabit Ethernet connection to the central router, could be called "layer 3
switches" because they only have Ethernet interfaces and must understand IP. It
would be more correct to call them access routers, where the router at the top is a
distribution router that connects to the Internet and academic networks' customer
access routers.The defining characteristics of LANs, in contrast to WANs (wide area networks),
include their higher data transfer rates, smaller geographic range, and lack of a
need for leased telecommunication lines. Current Ethernet or other IEEE 802.3
LAN technologies operate at speeds up to 10 Gbit/s. This is the data transfer
rate. IEEE has projects investigating the standardization of 100 Gbit/s, and
possibly 40 Gbit/s.
Campus Area Network (CAN)
A network that connects two or more LANs but that is limited to a specific and
contiguous geographical area such as a college campus, industrial complex, or a
military base. A CAN, may be considered a type of MAN (metropolitan area
network), but is generally limited to an area that is smaller than a typical MAN.
This term is most often used to discuss the implementation of networks for a
contiguous area.
Metropolitan Area Network (MAN)
A Metropolitan Area Network is a network that connects two or more Local Area
Networks or Campus Area Networks together but does not extend beyond the
boundaries of the immediate town, city, or metropolitan area. Multiple routers,
switches & hubs are connected to create a MAN.
Wide Area Network (WAN)
A WAN is a data communications network that covers a relatively broad
geographic area (i.e. one city to another and one country to another country) and
that often uses transmission facilities provided by common carriers, such as
telephone companies. WAN technologies generally function at the lower three
layers of the OSI reference model: the physical layer, the data link layer, and the
network layer.
Global Area Network (GAN)
Global area networks (GAN) specifications are in development by several groups,
and there is no common definition. In general, however, a GAN is a model for
supporting mobile communications across an arbitrary number of wireless LANs,
satellite coverage areas, etc. The key challenge in mobile communications is
"handing off" the user communications from one local coverage area to the next.
In IEEE Project 802, this involves a succession of terrestrial Wireless local area
networks (WLAN)
Internetwork
Two or more networks or network segments connected using devices that
operate at layer 3 (the 'network' layer) of the OSI Basic Reference Model, such as
a router. Any interconnection among or between public, private, commercial,
industrial, or governmental networks may also be defined as an internetwork.In modern practice, the interconnected networks use the Internet Protocol. There
are at least three variants of internetwork, depending on who administers and who
participates in them:• Intranet • Extranet • Internet Intranets and extranets may or may not have connections to the Internet. If
connected to the Internet, the intranet or extranet is normally protected from being
accessed from the Internet without proper authorization. The Internet is not
considered to be a part of the intranet or extranet, although it may serve as a
portal for access to portions of an extranet.
Intranet
An intranet is a set of interconnected networks, using the Internet Protocol and
uses IP-based tools such as web browsers, that is under the control of a single
administrative entity. That administrative entity closes the intranet to the rest of the
world, and allows only specific users. Most commonly, an intranet is the internal
network of a company or other enterprise.
Extranet
An extranet is a network or internetwork that is limited in scope to a single
organization or entity but which also has limited connections to the networks of
one or more other usually, but not necessarily, trusted organizations or entities
(e.g. a company's customers may be given access to some part of its intranet
creating in this way an extranet, while at the same time the customers may not be
considered 'trusted' from a security standpoint). Technically, an extranet may also
be categorized as a CAN, MAN, WAN, or other type of network, although, by
definition, an extranet cannot consist of a single LAN; it must have at least one
connection with an external network.
Internet
A specific internetwork , consisting of a worldwide interconnection of
governmental, academic, public, and private networks based upon the Advanced
Research Projects Agency Network (ARPANET) developed by ARPA of the U.S.
Department of Defense – also home to the World Wide Web (WWW) and
referred to as the 'Internet' with a capital 'I' to distinguish it from other generic
internetworks.Participants in the Internet, or their service providers, use IP Addresses obtained
from address registries that control assignments. Service providers and large
enterprises also exchange information on the reachability of their address ranges
through the Border Gateway Protocol (BGP).
Basic Hardware Components
All networks are made up of basic hardware building blocks to interconnect
network nodes, such as Network Interface Cards (NICs), Bridges, Hubs,
Switches, and Routers. In addition, some method of connecting these building
blocks is required, usually in the form of galvanic cable (most commonly Category
5 cable). Less common are microwave links (as in IEEE 802.11) or optical cable
("optical fiber").
Network Interface Cards
A network card, network adapter or NIC (network interface card) is a piece of
computer hardware designed to allow computers to communicate over a
computer network. It provides physical access to a networking medium and often
provides a low-level addressing system through the use of MAC addresses. It
allows users to connect to each other either by using cables or wirelessly.
Repeaters
A repeater is an electronic device that receives a signal and retransmits it at a
higher level or higher power, or onto the other side of an obstruction, so that the
signal can cover longer distances without degradation.
Hubs
A hub contains multiple ports. When a packet arrives at one port, it is copied to all
the ports of the hub. When the packets are copied, the destination address in the
frame does not change to a broadcast address. It does this in a rudimentary way,
it simply copies the data to all of the Nodes connected to the hub.
Bridges
A network bridge connects multiple network segments at the data link layer (layer
2) of the OSI model. Bridges do not promiscuously copy traffic to all ports, as hubs
do, but learns which MAC addresses are reachable through specific ports. Once
the bridge associates a port and an address, it will send traffic for that address
only to that port. Bridges do send broadcasts to all ports except the one on which
the broadcast was received.Bridges learn the association of ports and addresses by examining the source
address of frames that it sees on various ports. Once a frame arrives through a
port, its source address is stored and the bridge assumes that MAC address is
associated with that port. The first time that a previously unknown destination
address is seen, the bridge will forward the frame to all ports other than the one on
which the frame arrived.Bridges come in three basic types:1. Local bridges: Directly connect local area networks (LANs) 2. Remote bridges: Can be used to create a wide area network (WAN)
link between LANs. Remote bridges, where the connecting link is slower than the
end networks, largely have been replaced by routers. 3. Wireless bridges: Can be used to join LANs or connect remote stations
to LANs. Switches
Main article: Network switch
A switch is a device that performs switching. Specifically, it forwards and filters
OSI layer 2 datagrams (chunk of data communication) between ports (connected
cables) based on the Mac-Addresses in the packets.[6] This is distinct from a hub
in that it only forwards the datagrams to the ports involved in the communications
rather than all ports connected. Strictly speaking, a switch is not capable of routing
traffic based on IP address (layer 3) which is necessary for communicating
between network segments or within a large or complex LAN. Some switches are
capable of routing based on IP addresses but are still called switches as a
marketing term. A switch normally has numerous ports with the intention that most
or all of the network be connected directly to a switch, or another switch that is in
turn connected to a switch.
"Switches" is a marketing term that encompasses routers and bridges, as well as
devices that may distribute traffic on load or by application content (e.g., a Web
URL identifier). Switches may operate at one or more OSI layers, including
physical, data link, network, or transport (i.e., end-to-end). A device that operates
simultaneously at more than one of these layers is called a multilayer switch.Overemphasizing the ill-defined term "switch" often leads to confusion when first
trying to understand networking. Many experienced network designers and
operators recommend starting with the logic of devices dealing with only one
protocol level, not all of which are covered by OSI. Multilayer device selection is an
advanced topic that may lead to selecting particular implementations, but
multilayer switching is simply not a real-world design concept.
Routers
Routers are networking devices that forward data packets between networks
using headers and forwarding tables to determine the best path to forward the
packets. Routers work at the network layer of the TCP/IP model or layer 3 of the
OSI model. Routers also provide interconnectivity between like and unlike media
(RFC 1812). This is accomplished by examining the Header of a data packet,
and making a decision on the next hop to which it should be sent (RFC 1812)
They use preconfigured static routes, status of their hardware interfaces, and
routing protocols to select the best route between any two subnets. A router is
connected to at least two networks, commonly two LANs or WANs or a LAN and
its ISP's network. Some DSL and cable modems, for home use, have been
integrated with routers to allow multiple home computers to access the Internet.
classified by the network layer at which they operate according to basic reference
models considered as standards in the industry, such as the four-layer Internet
Protocol Suite model. While the seven-layer Open Systems Interconnection (OSI)
reference model is better known in academia, the majority of networks use the
Internet Protocol Suite (IP).
By scale
Computer networks may be classified according to the scale: Personal area
network (PAN), Local Area Network (LAN), Campus Area Network (CAN),
Metropolitan area network (MAN), or Wide area network (WAN).As Ethernet increasingly is the standard interface for networks, these distinctions
are more important to the network administrator than the user. Network
administrators may have to tune the network, to correct delay issues and achieve
the desired Quality of Service (QoS).
By connection method
Computer networks can also be classified according to the hardware technology
that is used to connect the individual devices in the network such as Optical fiber,
Ethernet, Wireless LAN, HomePNA, or Power line communication.Ethernets use physical wiring to connect devices. Often, they employ the use of
hubs, switches, bridges, and routers.Wireless LAN technology is built to connect devices without wiring. These devices
use a radio frequency to connect.
By functional relationship (Network Architectures)
Computer networks may be classified according to the functional relationships
which exist between the elements of the network, e.g., Active Networking,
Client-server and Peer-to-peer (workgroup) architectures.
By network topology
Computer networks may be classified according to the network topology upon
which the network is based, such as Bus network, Star network, Ring network,
Mesh network, Star-bus network, Tree or Hierarchical topology network, etc.Network Topology signifies the way in which intelligent devices in the network see
their logical relations to one another. The use of the term "logical" here is
significant. That is, network topology is independent of the "physical" layout of the
network. Even if networked computers are physically placed in a linear
arrangement, if they are connected via a hub, the network has a Star topology,
rather than a Bus Topology. In this regard the visual and operational
characteristics of a network are distinct; the logical network topology is not
necessarily the same as the physical layout.
By protocol
Computer networks may be classified according to the communications protocol
that is being used on the network. See the articles on List of network protocol
stacks and List of network protocols for more information. For a development of
the foundations of protocol design see Srikant 2004 [1] and Meyn 2007 [2]
Types of networks:
Below is a list of the most common types of computer networks in order of scale.
Personal Area Network (PAN)
A personal area network (PAN) is a computer network used for communication
among computer devices close to one person. Some examples of devices that
may be used in a PAN are printers, fax machines, telephones, PDAs or scanners.
The reach of a PAN is typically within about 20-30 feet (approximately 6-9
Meters).Personal area networks may be wired with computer buses such as USB[3] and
FireWire. A wireless personal area network (WPAN) can also be made possible
with network technologies such as IrDA and Bluetooth.
Local Area Network (LAN)
A network covering a small geographic area, like a home, office, or building.
Current LANs are most likely to be based on Ethernet technology. For example, a
library will have a wired or wireless LAN for users to interconnect local devices
(e.g., printers and servers) and to connect to the internet. All of the PCs in the
library are connected by category 5 (Cat5) cable, running the IEEE 802.3 protocol
through a system of interconnection devices and eventually connect to the internet.
The cables to the servers are on Cat 5e enhanced cable, which will support IEEE
802.3 at 1 Gbit/s.The staff computers (bright green) can get to the color printer, checkout records,
and the academic network and the Internet. All user computers can get to the
Internet and the card catalog. Each workgroup can get to its local printer. Note that
the printers are not accessible from outside their workgroup.
Typical library network, in a branching tree topology and controlled access to
resourcesAll interconnected devices must understand the network layer (layer 3), because
they are handling multiple subnets (the different colors). Those inside the library,
which have only 10/100 Mbps Ethernet connections to the user device and a
Gigabit Ethernet connection to the central router, could be called "layer 3
switches" because they only have Ethernet interfaces and must understand IP. It
would be more correct to call them access routers, where the router at the top is a
distribution router that connects to the Internet and academic networks' customer
access routers.The defining characteristics of LANs, in contrast to WANs (wide area networks),
include their higher data transfer rates, smaller geographic range, and lack of a
need for leased telecommunication lines. Current Ethernet or other IEEE 802.3
LAN technologies operate at speeds up to 10 Gbit/s. This is the data transfer
rate. IEEE has projects investigating the standardization of 100 Gbit/s, and
possibly 40 Gbit/s.
Campus Area Network (CAN)
A network that connects two or more LANs but that is limited to a specific and
contiguous geographical area such as a college campus, industrial complex, or a
military base. A CAN, may be considered a type of MAN (metropolitan area
network), but is generally limited to an area that is smaller than a typical MAN.
This term is most often used to discuss the implementation of networks for a
contiguous area.
Metropolitan Area Network (MAN)
A Metropolitan Area Network is a network that connects two or more Local Area
Networks or Campus Area Networks together but does not extend beyond the
boundaries of the immediate town, city, or metropolitan area. Multiple routers,
switches & hubs are connected to create a MAN.
Wide Area Network (WAN)
A WAN is a data communications network that covers a relatively broad
geographic area (i.e. one city to another and one country to another country) and
that often uses transmission facilities provided by common carriers, such as
telephone companies. WAN technologies generally function at the lower three
layers of the OSI reference model: the physical layer, the data link layer, and the
network layer.
Global Area Network (GAN)
Global area networks (GAN) specifications are in development by several groups,
and there is no common definition. In general, however, a GAN is a model for
supporting mobile communications across an arbitrary number of wireless LANs,
satellite coverage areas, etc. The key challenge in mobile communications is
"handing off" the user communications from one local coverage area to the next.
In IEEE Project 802, this involves a succession of terrestrial Wireless local area
networks (WLAN)
Internetwork
Two or more networks or network segments connected using devices that
operate at layer 3 (the 'network' layer) of the OSI Basic Reference Model, such as
a router. Any interconnection among or between public, private, commercial,
industrial, or governmental networks may also be defined as an internetwork.In modern practice, the interconnected networks use the Internet Protocol. There
are at least three variants of internetwork, depending on who administers and who
participates in them:• Intranet • Extranet • Internet Intranets and extranets may or may not have connections to the Internet. If
connected to the Internet, the intranet or extranet is normally protected from being
accessed from the Internet without proper authorization. The Internet is not
considered to be a part of the intranet or extranet, although it may serve as a
portal for access to portions of an extranet.
Intranet
An intranet is a set of interconnected networks, using the Internet Protocol and
uses IP-based tools such as web browsers, that is under the control of a single
administrative entity. That administrative entity closes the intranet to the rest of the
world, and allows only specific users. Most commonly, an intranet is the internal
network of a company or other enterprise.
Extranet
An extranet is a network or internetwork that is limited in scope to a single
organization or entity but which also has limited connections to the networks of
one or more other usually, but not necessarily, trusted organizations or entities
(e.g. a company's customers may be given access to some part of its intranet
creating in this way an extranet, while at the same time the customers may not be
considered 'trusted' from a security standpoint). Technically, an extranet may also
be categorized as a CAN, MAN, WAN, or other type of network, although, by
definition, an extranet cannot consist of a single LAN; it must have at least one
connection with an external network.
Internet
A specific internetwork , consisting of a worldwide interconnection of
governmental, academic, public, and private networks based upon the Advanced
Research Projects Agency Network (ARPANET) developed by ARPA of the U.S.
Department of Defense – also home to the World Wide Web (WWW) and
referred to as the 'Internet' with a capital 'I' to distinguish it from other generic
internetworks.Participants in the Internet, or their service providers, use IP Addresses obtained
from address registries that control assignments. Service providers and large
enterprises also exchange information on the reachability of their address ranges
through the Border Gateway Protocol (BGP).
Basic Hardware Components
All networks are made up of basic hardware building blocks to interconnect
network nodes, such as Network Interface Cards (NICs), Bridges, Hubs,
Switches, and Routers. In addition, some method of connecting these building
blocks is required, usually in the form of galvanic cable (most commonly Category
5 cable). Less common are microwave links (as in IEEE 802.11) or optical cable
("optical fiber").
Network Interface Cards
A network card, network adapter or NIC (network interface card) is a piece of
computer hardware designed to allow computers to communicate over a
computer network. It provides physical access to a networking medium and often
provides a low-level addressing system through the use of MAC addresses. It
allows users to connect to each other either by using cables or wirelessly.
Repeaters
A repeater is an electronic device that receives a signal and retransmits it at a
higher level or higher power, or onto the other side of an obstruction, so that the
signal can cover longer distances without degradation.
Hubs
A hub contains multiple ports. When a packet arrives at one port, it is copied to all
the ports of the hub. When the packets are copied, the destination address in the
frame does not change to a broadcast address. It does this in a rudimentary way,
it simply copies the data to all of the Nodes connected to the hub.
Bridges
A network bridge connects multiple network segments at the data link layer (layer
2) of the OSI model. Bridges do not promiscuously copy traffic to all ports, as hubs
do, but learns which MAC addresses are reachable through specific ports. Once
the bridge associates a port and an address, it will send traffic for that address
only to that port. Bridges do send broadcasts to all ports except the one on which
the broadcast was received.Bridges learn the association of ports and addresses by examining the source
address of frames that it sees on various ports. Once a frame arrives through a
port, its source address is stored and the bridge assumes that MAC address is
associated with that port. The first time that a previously unknown destination
address is seen, the bridge will forward the frame to all ports other than the one on
which the frame arrived.Bridges come in three basic types:1. Local bridges: Directly connect local area networks (LANs) 2. Remote bridges: Can be used to create a wide area network (WAN)
link between LANs. Remote bridges, where the connecting link is slower than the
end networks, largely have been replaced by routers. 3. Wireless bridges: Can be used to join LANs or connect remote stations
to LANs. Switches
Main article: Network switch
A switch is a device that performs switching. Specifically, it forwards and filters
OSI layer 2 datagrams (chunk of data communication) between ports (connected
cables) based on the Mac-Addresses in the packets.[6] This is distinct from a hub
in that it only forwards the datagrams to the ports involved in the communications
rather than all ports connected. Strictly speaking, a switch is not capable of routing
traffic based on IP address (layer 3) which is necessary for communicating
between network segments or within a large or complex LAN. Some switches are
capable of routing based on IP addresses but are still called switches as a
marketing term. A switch normally has numerous ports with the intention that most
or all of the network be connected directly to a switch, or another switch that is in
turn connected to a switch.
"Switches" is a marketing term that encompasses routers and bridges, as well as
devices that may distribute traffic on load or by application content (e.g., a Web
URL identifier). Switches may operate at one or more OSI layers, including
physical, data link, network, or transport (i.e., end-to-end). A device that operates
simultaneously at more than one of these layers is called a multilayer switch.Overemphasizing the ill-defined term "switch" often leads to confusion when first
trying to understand networking. Many experienced network designers and
operators recommend starting with the logic of devices dealing with only one
protocol level, not all of which are covered by OSI. Multilayer device selection is an
advanced topic that may lead to selecting particular implementations, but
multilayer switching is simply not a real-world design concept.
Routers
Routers are networking devices that forward data packets between networks
using headers and forwarding tables to determine the best path to forward the
packets. Routers work at the network layer of the TCP/IP model or layer 3 of the
OSI model. Routers also provide interconnectivity between like and unlike media
(RFC 1812). This is accomplished by examining the Header of a data packet,
and making a decision on the next hop to which it should be sent (RFC 1812)
They use preconfigured static routes, status of their hardware interfaces, and
routing protocols to select the best route between any two subnets. A router is
connected to at least two networks, commonly two LANs or WANs or a LAN and
its ISP's network. Some DSL and cable modems, for home use, have been
integrated with routers to allow multiple home computers to access the Internet.
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