ipcalc takes an IP address and netmask and calculates the resulting broadcast, network, Cisco wildcard mask, and host range. By giving a second netmask, you can design subnets and supernets. It is also intended to be a teaching tool and presents the subnetting results as easy-to-understand binary values.
Enter your netmask(s) in CIDR notation (/25) or dotted decimals (255.255.255.0). Inverse netmasks are recognized. If you omit the netmask ipcalc uses the default netmask for the class of your network.
Look at the space between the bits of the addresses: The bits before it are the network part of the address, the bits after it are the host part. You can see two simple facts: In a network address all host bits are zero, in a broadcast address they are all set.
The class of your network is determined by its first bits.
If your network is a private internet according to RFC 1918 this is remarked. When displaying subnets the new bits in the network part of the netmask are marked in a different color
The wildcard is the inverse netmask as used for access control lists in Cisco routers.
Do you want to split your network into subnets? Enter the address and netmask of your original network and play with the second netmask until the result matches your needs.
You can have all this fun at your shell prompt. Originally ipcalc was not intended for creating HTML and still works happily in /usr/local/bin/ :-)