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Table Of Contents  CertiGuide to Network+

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Appendix C:  Subnet Masks
(Page 1 of 2)

If the idea of subnet masks is alien to you, this section explains further. If after reading this appendix still has you lost, try going to http://www.learntosubnet.com . This is a free lecture that uses the Windows Media player.

Any IP address requires both an IP address that is 32 bits long, and in binary. For example,

00001010.00000000.00000000.0000001

The periods are there to make it simpler to read and are ignored by the computer.

To make it simpler than remembering 32 zeros and ones, IP addresses are called out in the range of 0-255. These are called octets. The example above becomes 10.0.0.1

IP addresses need a subnet mask to be complete. Standard IP addresses have common subnet masks. The classic subnet mask for the example above would be 255.0.0.0

A subnet mask breaks out the IP address into two pieces. The first is the network number, the second is a range of unique numbers set aside for invaded devices, called hosts. Most people have a last name and a first name. You may think of the subnet mask creating the last name, or family name. Since each network name must be unique, limiting the number of digits for the network name (last name) limits the number of unique networks. This can be extended by making the network name longer. However this comes at the expense of how many hosts (first names) that can exist on a network.

Long ago, in ancient Greece, the use of logic in discussion was formed. Working with binary numbers gives us what is called Boolean logic. There is four logic possibilities when working with binary numbers. They are AND, OR, XOR and NOT.

Figuring out what part of an IP address is the network name with the rest being the host name uses the Boolean logic AND. This is called the ANDing process. ANDing works as shown in Table 12.

Table 12

Variable A

(IP Address)

Variable B
(Subnet Mask)

Variable A AND
Variable B
(Combination)

0

0

0

0

1

0

1

0

0

1

1

1


In combining an IP address and subnet mask it looks like this:

00001010.00000000.00000000.0000001 = 10.0.0.1
11111111.00000000.00000000.0000000 = 255.0.0.0
nnnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhh n = Network Name, h=host name

Now, if your brain works like it does for your author of this section, this explanation makes as much sense as having the answer to the average retail price of a spool of green thread, in Paris. In other words, this makes no sense to me, at all.

If the math doesn't ring for you either, lets try a graphic thought to get the same idea. Pretend the ones are actually pipes that allow the IP number to flow down through to make a network name. And the zeros are little sponges that soak up the rest of the IP address preventing them from coming down and allowing them to be host names.

When viewed this way, it makes more sense (to me) and looks like this:

00001010.00000000.00000000.0000001 = 10.0.0.1
11111111.00000000.00000000.0000000 = 255.0.0.0

In the above 10.0.0.1 binary string….

00001010 = 10 = network name
00000000.00000000.0000000 Looks to me like sponges preventing binary from flowing though, allowing host names.

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Appendix D: Graphics
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