Network segmentation is a way of dividing a big computer network (we can consider a network as a Broadcast Domain) into subnetworks, each being a network segment or network layer. A big network can be segmented to smaller subnets using a Router.
1) Decrease in available Bandwidth: Large number of Broadcasts will reduce the available bandwidth of network links for normal traffic because the broadcast traffic is forwarded to all the ports in a switch. Every device in the broadcast domain wil receive the brodcast.
2) Decrease in processing power of computers: Since all the computers need to process all broadcast packets, a huge portion of the computer CPU power is spent on processing the broadcast packets. This will reduce the processing power of computers.broadcast domains, we can keep the local broadcast traffic local. Routers drop unwanted traffic originating from one network to pass through the router to reach another network, thus increasing the bandwidth available to each user.
Another benifits of network segmentation using Routers include
Media Transition: Routers are used to connect networks of different media types. For example, one of your network segment may be using Tokenring as LAN Standard (just as an example, Tokenring is out from industry long way back) and other network segment is using Ethernet as the LAN Standard. A Router can be used to connect these different LAN Standards.