Facts And Figures - article database

Calculating Host Bits and Determining the Subnet Mask In this scenario, you’ve been given a design requirement that no more than six computers exist on the same network (one application computer + ?ve additional computers). Remem- ber, the primary goal of a router is to stop broadcast traffic. And, in order to communicate from one subnet to another, you have to use a router. Thus, you have to determine the appropriate subnet MCTS Windows 7 Configuration mask to accommodate four hosts. The following is the default subnet mask for a Class C address:

The part in bold is referred to as the host portion of the subnet mask. And the eight 0s refer to eight individual bits. The way you can determine the number of hosts is by taking 2. raising it to the number of available host bits, and then subtracting 2 again to give room for the broadcast address and the network address. In this case, the amount of available hosts would be 28 ?2 = 254.

So, with that in mind, let’s look at the example where you need four hosts per subnet. The easiest way to do this is by seeing which power of 2 gives you at least two more addresses than you need and then use that. You do this to accommodate for the broadcast and net- work addresses. Here are the powers of 2:
21 = 2
22 = 4
23 = 8
24 = 16 25 = 32 26 = 64 27 = 128 28 = 256

As you can see, in order to accommodate at least two more than 4 (in this case four more), you need to use 2^3 bits. In total, this gives you 23 ?2 = 6 hosts .

So if you need three host bits to get six hosts, your network portion can take up all but 3 bits! That means the subnet mask for this problem is as follows:
11111111.11111111.11111111.11111000
The network portion is as follows:
11111111.11111111.11111111.11111 000
And the host portion is as follows:
11111111.11111111.11111111.11111 000
In shorthand, you would write this mask as /29, because it uses 29 bits. But the actual
value for this mask is 255.255.255.X, where X is the last octet in decimal. And the number
11111 in decimal happens to be 248. Thus, the mask for this problem is as follows: 255.255.255.248

Determining the Number of Subnetworks
The next portion of the problem asks you to determine whether you can accommodate Microsoft Exam 30 different subnetworks. This can be easily answered by examining the number of subnet bits available to you. Since you’re using a Class C address, you look at the network portion of the last octet only:
11111111.11111111.11111111 .11111 000

This is referred to as the subnet bits portion of the network portion of the subnet mask (that?s a lot to say at once). But in reality, it’s pretty easy to understand. This size grows and shrinks based on two factors. First, there is the address class. If you were using a Class A address, you?d look at all the networks bits in the second, third, and fourth octet combined. And if you were using a Class B address, you?d look at the third and fourth combined.

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