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You can expand a network with physical devices, such as routers and bridges, to add network segments. You can also use physical devices to divide a network into smaller sections to increase network efficiency. Network segments separated by routers are called subnets.
 

When creating subnets, you must split the network ID for the hosts in the subnets. Dividing the network ID used for communication on the Internet into smaller network IDs (based on the number of identified IP addresses) for a subnet is called subnetting a network. To identify the new network identifier for each subnet, you must use a subnet mask to specify which part of the IP address is to be used for the new network identifier of the subnet.

To find a host on a network, you can analyze the network ID of the host. Matching network identifiers indicate which hosts are on the same subnet. If the network identifiers are not identical, this indicates that they belong to different subnets and that you need a router to communicate between them.