OK, you have created your VLANs, which means you logically segmented your network at the data-link layer of the OSI model. You understand the reasons why you did this: better administration, more security, broadcast control, increased bandwidth, and it's much easier to administrate.
All those are excellent reasons to create VLANs, but now departments cannot communicate with each other. If you are in your own VLAN, you can talk to everyone in your VLAN, but that's it. What happens when you want to go out to the internet? Who is your default gateway? Think about it: you have four VLANs, which means four networks and your router may have one or two Ethernet ports on it. So, how do we accomplish communications with the outside world and between departments? The answer is inter-VLAN routing must be configured on a Layer 3 or network layer device.
Let...