Subnet Calculator

InternetPeriscope's Subnet Calculator can help you calculate subnet masks and ranges of IP addresses.

In this example, our user has just setup her company's office with a new ISP. The ISP has told her that her address range is She uses the Subnet Calculator tool to determine what this means. Later in the example, she uses the Subnet Calculator Tool to further subdivide her network in order to reduce network congestion from Broadcast traffic.

To run the Subnet Calculator Tool, she clicks on the Tools menu, and clicks on the "Subnet Calculator..." menu item, as shown in the figure below.
Tools menu

She enters her IP address, into the IP Address control. She then clicks once on the up-down control to set the number of network bits to "/25", the value her ISP specified.
Enter IP Address

The dialog above shows the following:

  • If she does not further subnet her IP address range, she should set the subnet mask on all of her computers to
  • Her ISP has given her part of a Class C address space.
  • The network address is Routing tables store the "route" to this network using the network address. Routing tables are used by software such as RIP and OSPF.
  • Without using a NAT server or proxy server, she can setup up to 126 machines with unique addresses (she will probably need to use one of these addresses as her router address, depending on her setup).
  • The broadcast address is If she sends a message to this address, it will be broadcasted to all of the machines in that subnet.
  • The first usable host address is Most admins assign this to the router that connects their site to their ISP. They then configure their client machines to use this address as a default gateway.
  • For this network, the administrator should only configure machines with IP addresses in the range -

The administrator wishes to further subnet her IP address range. She wishes to do this to decrease broadcast traffic. This will give her users faster network performance.

Currently, her company has two departments, each with 20 computers. She wishes to create a separate subnet for each of these departments, giving each department room for future growth. She plays around with the Subnet Calculator, and comes up with the following:

  • She will create two subnets, each with 30 IP addresses.
  • The first subnet will have the IP address range - She has decided to use the first address,, as the default gateway (router address) for this subnet.
  • Hosts on both networks will use the subnet mask