Subnet Calculator

The Subnet Calculator makes it easier to specify IP addresses and masks for routing or filtering.

The Subnet Calculator converts prefix lengths into corresponding dotted quad net masks. The Prefix Length is adjusted by clicking on the up/down arrows and will update the Mask field accordingly.

The Subnet Calculator shows the traditional (classful) IP address class in the upper right based on the IP Address field or previously selected prefix length. Press Return to specify the default non-subnetted mask for this class. If you use the prefix length stepper to override the traditional address class, the Subnet Calculator will use the override value until the prefix length matches the address class. You can force the prefix length to match the address class by closing and re-opening a Subnet Calculator window.

The Subnet Calculator shows the Network# and Host# portion of an IP address based on the Prefix Length. The network and host portion of the IP Address can be adjusted separately by clicking on the corresponding arrows. Once the correct values are shown, they can be dragged or copied to other input fields as needed.

The Subnet Calculator can determine the CIDR aggregates for a range of IP addresses or classful networks. Two IP addresses separated by a hyphen form an IP address range (example: After entering an address range, the Network# field will display the corresponding networks using CIDR aggregate notation (network prefix/length). Pressing the up/down arrows to the right of the Network# field will step through the CIDR aggregates needed to represent the corresponding network range. For example: network range can be represented by CIDR aggregates (4 class C networks) and (8 class C networks).

The Subnet Calculator shows the host address range for a CIDR aggregate (network prefix/length). Entering "10.0/27" for example will show the Host# as address range Notice the Host# address range ends with the subnet broadcast address (all 1's) even though this address should not be assigned to a host.

The Subnet Calculator determines the CIDR alignment based on the address class and right most 1-bit in the network prefix if an address ends in "/" with no length specified. Entering "146.111.96/" for example will show the Network# as

The Subnet Calculator will accept a decimal IP address as input and convert to a dotted quad. Entering "3232235521" will display IP address for example. This can used to help track down spammers who disguise their IP address in decimal form.

When you invoke one window from another, the corresponding data is automatically transferred.

A popup menu keeps a list of Recent Targets or history pre-loaded with some common values based on your network configuration. The contents of the target field are added to the list when a test to that target is invoked. If the recent targets menu becomes full (10 entries), the least recently used item will be removed. To add or remove an item, or clear the entire list, use the corresponding selections from the History menu.

You can open and use multiple Subnet Calculator windows at the same time.

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