Hub, Bridge, Router and Switch

Since I studied network technologies in college, I had been confused by many terms in network world. Recently, I faced these network terms again due to project requirement. So I did some studying again and summarized the most confusing terms here.

Network is a group of computers connected together in a way that information exchange among them is enabled.

Network Node is anything that is connected to the network as information sender/receiver. It is typically a computer, but may also be printer or CD-ROM tower.

Network Segment is a portion of a computer network wherein every node communicates using the same physical layer.

Hub works at the Layer 1 in the OSI network model. It broadcast packet from one port to all the other ports, thus makes devices connected by all ports to form one broadcast domain. In one broadcast domain, each network node receives all packets sent by other nodes. Hubs just extend the network segment to a larger scale, since it just does physical layer broadcasting.

Bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Rather than broadcasting packets to all ports, Bridges are capable of analyzing incoming data packets to determine which port should be used to send the packet to destination node, which is usually located in other network segment. In ethernet world, MAC address is used to do the destination port selecting. Bridges create new network segment, since they do switching at data link layer and isolate each broadcast domains.

Router works at layer 3(IP layer) to connect different subnets. It transmits logically addressed packets from their source toward their ultimate destination through intermediate nodes.

At conceptual level, Router Bridge do the same work - packet forwarding. The forwarding decision is based on two policies: routing(by routers) and briding(by bridges). Routing uses information encoded in device's address to infer its locatio on the network, while Bridging makes no assumptions about where the addresses are located and depends heavily on broadcasting to locate unknown addresses. Routing do the forwarding according to the destination node's network location, while Bridging do the forwarding according to the destination node's address.
Routing assumes that network addresses are structured and that similar addresses imply proximity within the network. Because structured addresses allow a single routing table entry to represent the route to a group of devices, structured addressing (routing, in the narrow sense) outperforms unstructured addressing (bridging) in large networks.

Switch is a broad and imprecise marketing term. This term commonly refers to a Network bridge that processes packet forwarding at the Data link layer (layer 2) of the OSI model. Switches that additionally process packet at the Network layer (layer 3 and above) are often referred to as Layer 3 switches, which is also used interchangeably with Router.


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