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All computers accessing the Internet have an "Internet Protocal address" (ip address).

IP Address
The ip address is a 32 bit number consisting of 4 sets of 8 bit binary numbers in decimal format separated by a period.

You can see your ip address (on Windows machines) by opening a command prompt and typing "ipconfig" (without the quotes of course and assuming the command is in your path - but that is a different problem).

Binary consists of 1's and 0's (two numbers so it can also be called "base 2").

An 8 bit binary number can have a maximum value of 255 and a minimum value of 0

Example: 11111111 = 255

Example ip address: 192.168.1.1

Which is the decimal form of: 11000000.10101000.00000001.000 00001

Subnet Mask
The ip address is logically divided into two parts: the "network address" and the "node address."

Way back in the early 1990's "the Internet" was facing a shortage of available ip addresses. Subnetting was devised to allow better utilization of the available addresses.

Dividing the ip address into network and node sections also allows for "traffic control" within the network.

How do we distinguish between the network and node portions of the address? -drum roll- With the subnet mask!

The subnet mask is also a binary number and defines the network portion of the ip address.

Some restrictions apply to the both the ip address and the subnet mask. Most notably addresses of all 1's or all 0's are not allowed (for routing reasons).

Example subnet mask: 255.255.255.0

Which equals: 11111111.11111111.11111111.000 00000

If we apply the above subnet to our example ip address:

192.168.001.001
255.255.255.000
-
192.168.001.000 = the network address
000.000.000.001 = the node address

A bitwise logical AND occurred between the dashes - which looked like this:
11000000.10101000.00000001.000 00001
11111111.11111111.11111111.000 00000
-
11000000.10101000.00000001.000 00000

All the 1+1 stay 1 and the 0+0 stay 0, 0+1 becomes 0 - feel free to ignore this ;-)

The subnet mask can also be represented by counting the bits.

Example 255.255.255.0 = 24

Therefore the network and subnet from above = 192.168.1.0/24

A network divided into 192.168.1.0/24 has 253 available "node" addresses (192.168.1.1 through 192.168.1.254).

Addresses 192.168.1.0 is the network address (which isn't available for use) and 192.168.1.255 is the "broadcast address" (which is used to send traffic to all of the nodes on a network).

Subnetting is all about breaking up "broadcast" networks and controlling traffic. Subnetting can get complicated, but this should serve as an introduction to subnet masks and subnetting...

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