CERTIFICATION OBJECTIVE 501 in Software

Printer PDF417 in Software CERTIFICATION OBJECTIVE 501

In this section you will walk through a subnetting example using a class A address The example is that you have an address range of 10000 and you would like to subnet, or divide, the network into two subnetworks known as subnets The physical network structure is shown in Figure 5-1 The concept of subnetting is to take some of the host bits from the subnet mask and use them as additional network bits, which will give you more networks With subnetting, you are taking additional host bits

FIGURE 5-1

and masking them ( agging them as network bits) by setting the bits to a 1 so that they are no longer representing the host ID portion of the subnet mask This creates additional networks but results in fewer hosts on the network To begin, we need to know how many host bits to take to make the desired number of networks A cheat way that does not involve much math is to look at the 8-bit binary table and then enable the bits that will give you the number of networks you are looking for For example, you want to divide a class A address of 10000 into two networks (be sure to remember that your goal is to have two networks), so you will enable the bits shown in the following table to come up with the decimal value of two Bit Value State

8 128 7 64 6 32 5 16 4 8 3 4 2 2 1 1 1

Once you have mathematically enabled the bits that will give you the number of networks you are looking for, the next step is from right to left to turn off any bits that are not being used up to the last bit you have enabled Once you have done that, you count the number of bits you have worked with from right to left In the example, you will simply need to disable the rst bit (far right side), and then count from right to left the number of bits that you have worked with In the example,

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there are two bits that have been manipulated, so the number of bits that you need to create two networks is two You can see the work in the following table Bit Value State

8 128 7 64 6 32 5 16 4 8 3 4 2 2 1 1 1 0

You can mathematically verify that you have the correct number of bits you want to work with by using this formula: 2masked bits 2 number of networks It was decided that two bits are needed to mask in the subnet mask to do the subnetting; in checking your work by lling in the formula, you should have 22 2 2 networks Having two networks was your goal, so you are on track! Now that you know that two bits must be masked to create the two subnets, the next step is to look at your IP range of 10000 and ask yourself, What is the default subnet mask of this address You should come up with 255000, because it is a class A address Once you know the default subnet mask, the next step is to break the subnet mask down into binary A default class A subnet mask in binary looks like the following: Decimal Binary

255 11111111 0 00000000 0 00000000 0 00000000

Once the subnet mask has been written out in binary, this will be the work area to calculate the new addresses of the two subnets The rst thing you need to do is steal, or mask, two bits from the host ID portion of the subnet mask and make them network bits by setting them to a 1 state Remember, the reason for taking two bits is because of your calculation earlier Because the last three octets of this subnet mask represent the host bits of a class A address, work from left to right and mask two additional bits to get the new binary table that follows Decimal Binary

255 11111111 0 11000000 0 00000000 0 00000000

Notice that the two bits that are now enabled are in the second octet Also notice that you always work from left to right, except for that little cheat method you saw earlier for calculating the number of bits required to mask That cheat is just to get