Introduction to Data Communications
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In the introduction to subnetting, the portion of the IP address that corresponded to the network portion was easily identified as being the portion of the subnet mask that corresponded to the decimal number 255. This is really only for convenience for the dot decimal format of the IP address. In actual fact, the IP address is a 32 bit address and doesn't have byte "boundaries" as implied by the dot decimal notation. For example:

	192.168.1.0  =  1100 0000 1010 1000 0000 0001 0000 0000

This means that the portion of the subnet mask that corresponds to the network address can be further broken down on the host bit positions.

A Class C network address of 192.168.1.0 has 254 hosts available to it. If your network consisted of 4 different physical locations each with a maximum of 50 hosts, then subnetting the network would be required. The locations could be different buildings or cities.

Bit 7 and bit 6 of the host portion can be used to describe the network portion of the subnet. The subnet masking would be:

	1111 1111 1111 1111 1111 1111 1100 0000  =  255.255.255.192

The 4 subnets for Class C address 192.168.1.0 would be 192.168.1.0, 192.168.1.64, 192.168.1.128 and 192.168.1.192 with the following range of IP addresses:

	Subnet			Host Range		Broadcast address

	192.168.1.0	192.168.1.1 to 192.168.1.62	192.168.1.63

	192.168.1.64	192.168.1.65 to 192.168.1.126	192.168.1.127

	192.168.1.128	192.168.1.129 to 192.168.1.190	192.168.1.191

	192.168.1.192	192.168.1.193 to 192.168.1.254	192.168.1.255

In this manner, a router with 4 interfaces could be configured with subnet masks of 255.255.255.192 to allow subdividing the Class C network into 4 smaller networks.

Theoretically, all of the host bits up to bit 1 and 0 can be used to make up to 64 subnets of 2 hosts each. In this case, 128 IP addresses would be lost to the network IP address and the broadcast IP address. The following table lists the number of hosts and networks that can implemented using subnet masking for a Class C network:

	Subnet mask	Number of subnets	Number of hosts per subnet

	255.255.255.128		2			126

	255.255.255.192		4			62

	255.255.255.224		8			30

	255.255.255.240		16			14

	255.255.255.248		32			6

	255.255.255.252		64			2

The above example is based on subnetting a Class C network. Subnetting can get extremely complicated if you are subnetting assigned IP addresses that are in the middle of a Class C network such as when rented from an ISP. Fortunately, there are many Subnet Mask Calculators available for download off the Internet that are designed to determine the correct subnet mask for your network.

Introduction to Data Communications
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