Why segment LANs?

Why segment LANs? 
4.3.5 Highlight that there are two primary reasons for segmenting a LAN. The first is to isolate traffic between segments. The second reason is to achieve more bandwidth per user by creating smaller collision domains. By this stage, students have heard of this term several times but instructors are encouraged to make sure that students understand the difference between collision and broadcast domains. The three figures are particularly useful.

This page will explain the two main reasons to segment a LAN.
There are two primary reasons for segmenting a LAN. The first is to isolate traffic between segments. The second reason is to achieve more bandwidth per user by creating smaller collision domains.
Without LAN segmentation, LANs larger than a small workgroup could quickly become clogged with traffic and collisions.
LAN segmentation can be implemented through the utilization of bridges, switches, and routers. Each of these devices has particular pros and cons.
With the addition of devices like bridges, switches, and routers the LAN is segmented into a number of smaller collision domains. In the example shown, four collision domains have been created.
By dividing large networks into self-contained units, bridges and switches provide several advantages. Bridges and switches will diminish the traffic experienced by devices on all connected segments, because only a certain percentage of traffic is forwarded. Bridges and switches reduce the collision domain but not the broadcast domain.
Each interface on the router connects to a separate network. Therefore the insertion of the router into a LAN will create smaller collision domains and smaller broadcast domains. This occurs because routers do not forward broadcasts unless programmed to do so.
A switch employs "microsegmentation" to reduce the collision domain on a LAN. The switch does this by creating dedicated network segments, or point-to-point connections. The switch connects these segments in a virtual network within the switch.
This virtual network circuit exists only when two nodes need to communicate. This is called a virtual circuit as it exists only when needed, and is established within the switch.
The next page will discuss microsegmentation.