|  | 10.1 | Importance of a Network Layer
| 10.1.2 | Segmentation and autonomous systems

There are two primary reasons why multiple networks are necessary - the growth in size of each network and the growth in the number of networks. When a LAN, MAN, or WAN grows, it may become necessary or desirable for network traffic control to break it up into smaller pieces called network segments (or just segments). This results in the network becoming a group of networks, each requiring a separate address.

There are already a vast number of networks in existence; separate computer networks are common in offices, schools, companies, businesses, and countries. It is convenient to have these separate networks (or autonomous systems, if each is managed by a single administration) communicate with each other over the Internet. However, they must do it with sensible addressing schemes and appropriate internetworking devices. If not, the network traffic flow would become severely clogged, and neither the local networks, nor the Internet, would function.

An analogy that might help you understand the need for network segmentation is to imagine a highway system and the number of vehicles that use it. As the population in the areas surrounding the highways increases, the roads become burdened with too many vehicles. Networks operate much in the same way. As networks grow, the amount of traffic grows. One solution might be to increase the bandwidth, much the same as increasing the speed limits of, or adding lanes to, the highways. Another solution might be to use devices that segment the network and control the flow of traffic, the same way a highway would use devices such as stoplights to control the movement of traffic.