To pass your Network+ certification exam, you must know the differences between hubs, repeaters, bridges, and switches. The differences are straightforward for the most part, so let's examine the basic purpose of these devices.
Hubs and repeaters both work at the Physical layer of the OSI model. They do not help to direct traffic toward the proper destination, as routers and switches do. Their purpose in the network is simply to make the signal strong enough to reach its destination, by regenerating the signal. Hubs and repeaters both take an incoming data signal and make a clean, strong copy of the signal. Otherwise, the signal would suffer from attenuation, the gradual weakening of a signal as it gets farther and farther away from the source.
Repeaters are just about extinct in today's networks, because they only have one input port and one output port. Naturally, this means we could only regenerate a single data signal. Hubs are really just multiport repeaters, but their multiple ports give them some value in today's networks.
By using a hub to connect hosts, we're left with one collision domain. Any data sent by these hosts is subject to colliding with data sent by another host. We'll talk more about collision domains in another CompTIA tutorial, but keep in mind that hubs do not break them up.
Using a hub also results in one broadcast domain, since a broadcast sent by any of these hosts will be heard by all other hosts.
Bridges and switches work at the Data Link layer of the OSI model, making them "Layer Two" devices. Both read MAC addresses to create a MAC address table, which allows the switches to help send frames to their proper destination. You see very few bridges in today's networks, especially with the advent of Layer Three switches.
It's not that bridges are bad at their job, it's just that switches are better. They can operate at higher speeds and offer more options, especially now that many new switches can perform routing as well as switching. (Layer Three switches used to be called brouters, but as the usage of bridges has faded, so has the use of this term.) There are two rather confusing facts regarding switches that confuse many newcomers, and I want to mention them again here. First, MAC addresses are also called physical addresses because they physically exist on the network interface card (NIC). They have nothing to do with the Physical layer of the OSI model.
Second, you're going to hear several different names for the MAC address table throughout your career and your networking certification studies.
MAC address table bridging table switching table physical address table Layer Two address table They're all the same thing! But no matter what you call it, you should know how switches build this table and use it to determine the appropriate action to take with incoming frames. A switch is either going to forward, flood, or filter a frame, depending on whether it knows how to reach the destination MAC address of the frame. We'll look at that process in the next CompTIA Network+ certification exam tutorial! See you then!
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Article added to SearchWarp.com on Monday, October 23, 2006 View other articles written by Chris Bryant CCIE 12933(14,375)
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