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Physical And Virtual LANs

Earning your Cisco CCENT certification and passing the 640-822 exam demands that you master networking basics, and part of that is knowing how to work with both physical and virtual Local Area Networks (LANs).

A physical LAN is a collection of PCs, cables, network cards, and other network hardware - switches, routers, printers, and servers, for example. When we refer to the physical network, you're rarely going to hear anyone actually say "physical LAN". When someone mentions setting up a LAN, they're referring to the physical devices themselves.

So.... what the heck is a "virtual LAN"? The name seems to suggest some kind of imaginary network, which isn't quite true, since the physical LAN devices will also be part of the virtual LAN ("VLAN"). What we're actually doing is logically dividing the physical network into smaller, logical networks - and these smaller, logical groups are called VLANs.

Let's look at an example that will illustrate the concept and help explain why we would create VLANs in the first place. Let's say we have ten PCs connected to the same switch. As I explained in an earlier Cisco CCENT exam tutorial, this can lead to the transmission of unnecessary broadcasts. If one PC sends a broadcast, by default the switch will forward a copy of that broadcast frame to all the other PCs.

Most likely, not all of the other PCs need a copy of that broadcast. We can logically group these PCs into VLANs, and when a member of one VLAN sends a broadcast, only the other members of the same VLAN will receive the broadcast.

For example, we could create three VLANs in this situation by placing PC1, PC2, and PC3 into VLAN 1; PC4, PC5, and PC6 would go into VLAN 2; finally, PCs 7, 8, 9, and 10 could be placed into VLAN 3. When PC1 sends a broadcast, the switch will send a copy of that broadcast frame only to PC2 and PC3 - the other members of that VLAN. The other six PCs will not get a copy of the frame!

As you'll learn in networking, there's always a catch. In this scenario, not only will PCs in VLANs 2 and 3 not receive broadcasts send by PCs in VLAN1, but they'll be unable to receive any kind of traffic from those other hosts. By default, inter-VLAN traffic is not possible on a switch; an OSI model Layer 3 device must get involved. We'll talk about how to enable inter-VLAN traffic in the next installment of my Cisco CCENT exam training series!

About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the CCNA exam with Chris Bryant!