BRIDGES, ROUTERS, SWITCHES, AND HUBS

Network devices such as routers, switches, bridges, and hubs connect computing systems and networks together. They are responsible for productive network functionality, backbone support, and the proper forwarding of information to other networks. In short, without them, there would be little or no network connectivity and minimal network security at best.

As the number of computers or nodes on a network increases, a network requires specialized equipment to expand its length, direct its flow of traffic, and provide a centralized location for troubleshooting and maintenance. Next, we will discuss the functionality of each of these devices.

Bridges

Bridges are hardware devices that operate at the MAC sublayer of the OSI reference model’s data link layer. Bridges are used to segment or separate LANs. Separating a larger network into smaller, more manageable segments can improve network performance and provide a way to isolate network bottlenecks.

A bridge reads the MAC hardware address that is stored in the NIC card of every computer or node installed on either side of the bridge. The bridge knows where all the computers are on the network and can forward information to a particular computer by the use of its NIC MAC address. Let’s say you are sitting at a computer that resides on network segment number 1. You want to send Brian, whose computer is located on network segment number 2, a Word document. There is a bridge that separates you on network segment number 1 from Brian on network segment number 2. The bridge can identify both of your computers by their respective NIC’s MAC addresses. Therefore, when you send a Word document to Brian, it is forwarded to his network segment by the use of the bridge.

Bridges can provide the following services:

• Reduce network traffic that results from too many computers being attached to a network.

• Connect different types of media connections, such as coaxial cable and twisted-pair cable.

• Expand the length of a network segment.

• Connect different network typologies together, such as Token Ring and Ethernet.

Although bridges serve their primary purpose, they are limited in their capabilities. If a destination’s MAC address is not found in a bridge’s internal table, the bridge will proliferate, or broadcast (pass traffic) to all network segments. This can result in a broadcast storm that can slow or take down a network.

As networks grew larger, the demand increased for a more intelligent device that could handle an increasing number of attached computer nodes and direct network traffic in a more efficient manner. The router was technology’s answer to this demand.

Routers

A router is another device that connects different network segments; but unlike a bridge, a router does not use a computer’s MAC address to forward information. Instead, a router operates at the network layer of the OSI reference model and has the ability to forward information based on a network or an individual computer’s TCP/IP address. This allows a router to connect entirely separate networks and to filter information to the proper network or network segment. In other words, a router has the ability to send a request to a specific location without broadcasting to all the other computer nodes on a network or network segment.

Routers are very intelligent. They hold sophisticated routing tables and have the ability to remember previous connections that were used as pathways from one computer node to another. Routers can actually decide which path is most efficient for a packet of information to take in order to reach its final destination.

Routers are primarily used for the following tasks:

• Provide filtering of packets and reduce broadcast storms.

• Segment networks into smaller, more manageable pieces.

• Provide a network security layer between separate networks (a firewall).

Routers use specialized protocols, such as Internet Control Message Protocol (ICMP), Open Shortest Path First (OSPF), and Routing Information Protocol (RIP), to communicate with each other and carry out their advanced functions.

Switches

A switch is a network device similar to a router; it chooses certain paths or routes in a network on which to send data. A switch is not a router, although a switch can contain router functionality. Most modern switches can operate at both the data link and network layers of the OSI reference model. A switch that has the ability to operate at the network layer is known as a layer 3 or IP switch. Switches can connect networks and subnetworks comprised of the same or different cable types. They can send units of data (packets) faster than most routers based on digital packet-switching technology. Switches typically connect LAN segments that use the same protocol.

Hubs

A hub is a network device that acts as a central point used to connect computers together. In network terms, a hub is a simple connection device that sends all data packets to all connected systems. A basic hub operates at the physical layer of the OSI reference model. Generally speaking, in a basic networking set-up, if the hub goes down or becomes dysfunctional, all systems connected to the hub will be unable to communicate with each other.