13.5 The virtual local area network (VLAN)

You’ll have noticed the VLAN in the interfaces list. This is a bit beyond the scope of this course, but as we will see [it] while we’re working with Packet Tracer Anywhere I should say something about it.

VLAN stands for ‘virtual local area network’. To understand what a VLAN is, imagine that an organisation has several local area networks, or LANs. Each LAN serves a different set of offices. You can see here that PC0 and PC1 belong to a LAN for the Finance Department. Suppose we want PC3 to be part of the Finance Department LAN, but it’s a long way from the Finance Department, maybe in another building.

We can’t easily run a cable from PC3 to Switch0, which is the obvious way to make it part of the Finance Department’s LAN. But PC3 is near to Switch1. Using a virtual local area network, or VLAN, it’s possible for PC3 to be connected to Switch1 and yet be part of the Finance Department’s LAN. PC2, though, can remain part of a different local network from the Finance Department’s. Similarly, with VLANs, Switch0 doesn’t have to be used solely by the Finance Department. It can be used by other LANs too. VLANs allow you to create LANs that are not defined by the networking infrastructure of switches and their attached networks. Sometimes this is expressed by saying that ‘the VLANs are based on logical connections rather than physical connections’. This makes it sound as though not using VLANs is illogical. What people mean when they use the word ‘logical’ like this is that VLANs allow you to create LANs based on their purpose, rather than on their physical location or connections. A VLAN needs an IP network address, which is why it appears in the show ip interface brief command.