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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 identifier for the hosts in the subnets. Splitting the network identifier used for Internet communications into smaller (based on the number of IP addresses identified) network identifiers for a subnet is called subnetting a network. To then identify the new network identifier for each subnet, you must use a subnet mask to specify which part of the IP address should be used for the new network identifier of the subnet.

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

In a TCP/IP environment, segments separated by routers are referred to as subnets. All computers belonging to a subnet have the same network identification in the IP addresses. Each subnet requires a different network identifier for communication with other subnets.

Based on the network identifier, subnets define the logical parts of a network. Computers in different subnets have to forward the communication via routers.