Test Your Firewall | IP Network Calculator | Cisco Password Decoder

Use this utility to determine network addresses, CIDR values, IP address ranges, and broadcast addresses.

IP Address: Netmask or CIDR:

So how is this done?

(This is a long and ugly explanation. I'll fix it up over time...)

An IP address is a set of four (4) bytes that identify a unique host address. Those four bytes are most commonly displayed as four decimal numbers separated by a period ("."). This representation is called dotted-quad notation.

Example IP Address:

Each byte contains eight (8) bits. The computer sees these values as binary numbers.


IP4 Networks are set up to use four one-byte values to figure out how to move information from one place to another. (8 bits in a byte, so 32 bits total)


So... the largest number that we can represent using eight binary digits is 255.

In our example network, the first "10" is fixed and each "x" can be any number from 0-255.

In the network, all information destined for computers or devices with an IP address of 10.10.10.x would be delivered to the same network space. In this case, "10.10.10" is fixed, and "x" can be any number from 0-255.

The Netmask defines the number of bits that CANNOT be used to represent a host IP address. In our first example, only the first value is fixed "10". That leaves three "x" values that can be used to uniquely identify a single computer. In other words, one of the dottect quad values ("10") CANNOT be used to uniqely identify a computer because all addresses on that network start with the same value ("10").

Would could have a computers identified as "", "", "", etc. We would NOT put a computer identifed as "" on that network, because it doesn't start with a "10".

If we cannot use the first value in our dotted quad, then we have only 3 values of our dotted quad remaing to uniquely identify computers. A network mask is effectively the binary digits that are NOT available for us to use to uniquely identify computers.


Second Example:

Last Example:

The actual number of usable IPs for computers is slightly different than described above. You can google "network address" and "broadcast address" if you really want to...