NetworkSense

 How to Subnet a Network? | PART 3 | IP Addressing to IP Subnetting

Subnetting can be useful in a variety of ways, including simplifying network administration, enabling you to use different physical media such as Ethernet and FDDI, and adding a layer of security to your network. The most common use of subnetting is to control network traffic.

Subnetting is done by borrowing host bits and using them as network bits. To begin, let's look at our ABC company network address (192.168.1.0) and its subnet mask (255.255.255.0) as expressed in binary. Notice that the address bits that have corresponding mask bits set to 1 represent the network address. Address bits that have corresponding mask bits set to 0 represent the individual host address.

Network address	11000000	10101000	00000001	00000000
Subnet mask	11111111	11111111	11111111	00000000

With this address, the bits from octets 1, 2, and 3 are used to identify the network portion of the address. However, we could subnet our network by borrowing bits from the fourth octet. To do so, we must take bits consecutively from left to right. In the following table, we borrow the bit with a value of 128.. 

Network address	11000000	10101000	00000001	00000000
Subnet mask	11111111	11111111	11111111	10000000

This changes our subnet mask. Instead of 255.255.255.0, it is now 255.255.255.128. The more host bits you use for subnets, the more subnets you have available. However, as more subnets are created, the less host addresses are available per subnet. In the following table, we borrow both the 128 and the 64 bit. We now have only 6 bits left for host addresses, and our mask is now 255.255.255.192.

Network address	11000000	10101000	00000001	00000000
Subnet mask	11111111	11111111	11111111	11000000

Notice also that we use the fourth octet to subnet a Class C network. Because this document and the Subnet game refer only to Class C networks, the remainder of this document will focus only on the fourth octet. Once you understand IP subnetting for Class C networks, it will be much easier for you to understand IP subnetting for any network.

So, back to our fourth octet, exactly what network and host addresses can we create by borrowing these two bits and thus applying this .192 mask?

Network address	11000000	10101000	00000001	00000000
Subnet mask	11111111	11111111	11111111	11000000

Let's look first at the subnets. We have two digits with which to create subnets.

Network address	11000000	10101000	00000001	00000000
Subnet mask	11111111	11111111	11111111	11000000

Looking at the bit value chart, it's easy to see that those two digits can be . . .

00, which equals 0

Bit	0	0	0	0	0	0	0	0
Bit value	128	64	32	16	8	4	2	1

01, which equals 64

Bit	0	1	0	0	0	0	0	0
Bit value	128	64	32	16	8	4	2	1

10, which equals 128

Bit	1	0	0	0	0	0	0	0
Bit value	128	64	32	16	8	4	2	1

or 11, which equals 192

Bit	1	1	0	0	0	0	0	0
Bit value	128	64	32	16	8	4	2	1

So we have created the following subnets:

192.168.1.0

192.168.1.64

192.168.1.128

192.168.1.192

That means that these numbers (0, 64, 128, and 192) can no longer be host addresses because they are now subnet addresses. Each subnet, like any network, must have a broadcast address as well. The broadcast address is the last address on the network, so on our first subnet, 192.168.1.0, available host addresses are 192.168.1.1 through 192.168.1.62. The 192.168.1.63 is the broadcast address and 192.168.1.64 is the address of the next subnet. Remember that before we subnetted our network, we could use all 8 bits of the 4th octet for host addresses. That gave us 254 host addresses. We now have 62 for each of our 4 subnets, or 248 total. The following table shows the complete results of borrowing two host bits to subnet our
Class C network.