What is Subnet?

A subnet, or subnetwork, is a logical subdivision of an IP network into two or more network segments. With subnets, large networks can be subdivided into smaller, more efficient subnetworks to meet connectivity and security requirements. The simplest subnet, a point-to-point subnet, connects two devices. A data center subnet connects many devices or groups of users, which can be in one or multiple locations.

A Class A, B, or C network can be subnetted. (There are five different classes of networks, A to E, but classes D and E are reserved.) The subnetting process allows the administrator to divide a Class A, Class B, or Class C network number into smaller portions. The subnets can be also be subnetted into sub-subnets.

Subnet masks define a range of IP addresses that can be used in a network and to designate subnetworks. A subnet mask makes it possible to identify which part of an IP address is reserved for the network and which part is available for host use. Like IP addresses, a subnet mask contains four bytes (32 bits) and is often written using the same, number/decimal format.

The subnet mask is used by the TCP/IP protocol to determine whether a host is on the local subnet or a remote network. With a local subnet, the device will send a request to retrieve the hardware address of the system to communicate over the data-link layer. For addresses on remote networks, devices route packets to the gateway for that network.

Subnets provide a number of benefits, including:

  • Reduce network congestion
  • Improve network performance
  • Maximize the number of permitted hosts on a network
  • Provide access to a network remotely without having to open the entire network
  • Enable point-to-point links or subnetworks to support a few devices
  • Enhance security by isolating segments of a network (e.g., finance, sales, legal)