Editing Network topology (section)

==Nodes==
{{Main|Node (networking)}}

Network nodes are the points of connection of the transmission medium to transmitters and receivers of the electrical, optical, or radio signals carried in the medium. Nodes may be associated with a computer, but certain types may have only a microcontroller at a node or possibly no programmable device at all. In the simplest of serial arrangements, one [[RS-232]] transmitter can be connected by a pair of wires to one receiver, forming two nodes on one link, or a Point-to-Point topology. Some protocols permit a single node to only either transmit or receive (e.g., [[ARINC 429]]). Other protocols have nodes that can both transmit and receive into a single channel (e.g., [[CANbus|CAN]] can have many transceivers connected to a single bus). While the conventional [[Systems engineering|system]] building blocks of a [[computer network]] include [[network interface controller]]s (NICs), [[repeater]]s, [[Ethernet hub|hubs]], [[Network bridge|bridges]], [[Network switch|switches]], [[Router (computing)|routers]], [[modem]]s, [[Gateway (telecommunications)|gateways]], and [[Firewall (computing)|firewalls]], most address network concerns beyond the physical network topology and may be represented as single nodes on a particular physical network topology.

<!-- The following headings of Computer networking equipment are now possibly partially depreciated in this article, having been copied from [[Computer network]]. While these are indeed computer network building blocks, most provide services above the Physical Layer. Some headings can be modified to be inclusive of non-internet or non-Ethernet technologies specifically to address how they are relevant to  (e.g., repeaters have long use in serial protocols, USB depends on Hubs). -->
===Network interfaces===
[[File:ForeRunnerLE 25 ATM Network Interface (1).jpg|thumb|right|alt=A network interface circuit with port for ATM|An [[Asynchronous Transfer Mode|ATM]] network interface in the form of an accessory card. A lot of network interfaces are built-in.]]
A [[network interface controller]] (NIC) is [[computer hardware]] that provides a computer with the ability to access the transmission media, and has the ability to process low-level network information. For example, the NIC may have a connector for accepting a cable, or an aerial for wireless transmission and reception, and the associated circuitry.

The NIC responds to traffic addressed to a [[network address]] for either the NIC or the computer as a whole.

In [[Ethernet]] networks, each network interface controller has a unique [[MAC address|Media Access Control]] (MAC) address—usually stored in the controller's permanent memory. To avoid address conflicts between network devices, the [[Institute of Electrical and Electronics Engineers]] (IEEE) maintains and administers MAC address uniqueness. The size of an Ethernet MAC address is six [[Octet (computing)|octets]]. The three most significant octets are reserved to identify NIC manufacturers. These manufacturers, using only their assigned prefixes, uniquely assign the three least-significant octets of every Ethernet interface they produce.

===Repeaters and hubs===
A [[repeater]] is an [[Electronics|electronic]] device that receives a network [[signal]], cleans it of unnecessary noise and regenerates it.  The signal may be reformed or [[retransmission (data networks)|retransmitted]] at a higher power level, to the other side of an obstruction possibly using a different transmission medium<!-- technically bridge-to-bridge -->, so that the signal can cover longer distances without degradation. Commercial repeaters have extended [[RS-232]] segments from 15 meters to over a kilometer.<ref>U.S. Converters, [http://www.usconverters.com/rs232-repeater-extender RS232 Repeater]</ref> In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. With fiber optics, repeaters can be tens or even hundreds of kilometers apart.

Repeaters work within the physical layer of the OSI model, that is, there is no end-to-end change in the physical protocol across the repeater, or repeater pair, even if a different physical layer may be used between the ends of the repeater, or repeater pair. Repeaters require a small amount of time to regenerate the signal. This can cause a [[propagation delay]] that affects network performance and may affect proper function. As a result, many network architectures limit the number of repeaters that can be used in a row, e.g., the Ethernet [[5-4-3 rule]].

A repeater with multiple ports is known as hub, an [[Ethernet hub]] in Ethernet networks, a [[USB hub]] in USB networks.
* [[USB]] networks use hubs to form tiered-star topologies.
* Ethernet hubs and repeaters in LANs have been mostly obsoleted by modern [[network switch|switches]].

===Bridges===
A [[network bridge]] connects and filters traffic between two [[network segment]]s at the [[data link layer]] (layer 2) of the [[OSI model]] to form a single network. This breaks the network's collision domain but maintains a unified broadcast domain. Network segmentation breaks down a large, congested network into an aggregation of smaller, more efficient networks.

Bridges come in three basic types:
*Local bridges: Directly connect LANs
*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 with routers.
*Wireless bridges: Can be used to join LANs or connect remote devices to LANs.

===Switches===
A [[network switch]] is a device that forwards and filters [[OSI layer 2]] [[datagram]]s ([[Frame (networking)|frames]]) between [[Computer port (hardware)|ports]] based on the destination MAC address in each frame.<ref>{{cite web
|url=http://www.webopedia.com/TERM/s/switch.html
|title=Define switch.
|date=September 1996
|website=Webopedia
|access-date=April 8, 2008
}}</ref>
A switch is distinct from a hub in that it only forwards the frames to the physical ports involved in the communication rather than all ports connected. It can be thought of as a multi-port bridge.<ref>{{cite web|url=http://compnetworking.about.com/cs/internetworking/g/bldef_bridge.htm|title=What bridge devices and bridging do for computer networks|access-date=2017-10-24|archive-date=2012-04-20|archive-url=https://web.archive.org/web/20120420022720/http://compnetworking.about.com/cs/internetworking/g/bldef_bridge.htm|url-status=dead}}</ref> It learns to associate physical ports to MAC addresses by examining the source addresses of received frames. If an unknown destination is targeted, the switch broadcasts to all ports but the source. Switches normally have numerous ports, facilitating a star topology for devices, and cascading additional switches.

[[Multi-layer switch]]es are capable of routing based on layer 3 addressing or additional logical levels. The term ''switch'' is often used loosely to include devices such as routers and bridges, as well as devices that may distribute traffic based on load or based on application content (e.g., a Web [[URL]] identifier).

===Routers===
[[File:Adsl connections.jpg|thumb|right|A typical home or small office router showing the [[ADSL]] telephone line and [[Ethernet]] network cable connections]]
A [[Router (computing)|router]] is an [[internetworking]] device that forwards [[Packet (information technology)|packets]] between networks by processing the routing information included in the packet or datagram (Internet protocol information from layer 3).  The routing information is often processed in conjunction with the [[routing table]] (or forwarding table).  A router uses its routing table to determine where to forward packets. A destination in a routing table can include a [[Black hole (networking)|black hole]] because data can go into it, however, no further processing is done for said data, i.e. the packets are dropped.

===Modems===
[[Modem]]s (MOdulator-DEModulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless. To do this one or more [[carrier signal]]s are [[modulated]] by the digital signal to produce an [[analog signal]] that can be tailored to give the required properties for transmission. Modems are commonly used for telephone lines, using a [[digital subscriber line]] technology.

===Firewalls===
A [[Firewall (computing)|firewall]] is a network device for controlling network security and access rules. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in [[cyber attack]]s.