150
Chapter 3
Internet Protocols
That wasn't all that hard, was it? Hopefully, you understood what I was
trying to show you. The example I presented only used two subnet bits, so
what if you had to subnet using 9, 10, or even 20 subnet bits? Try that with
the binary method and see how long it takes you. Since the Cisco CCNA
exam gives you just over a minute for each question, it's really important to
know how much time you'll spend on a subnetting question. That's why
committing as much as possible to memory as I suggested earlier in the chap-
ter is vital. Using the binary method can take you way too long and you
could fail the exam even if you know the material!
This is why I'm going to teach you an alternate method of subnetting that
makes it easier to subnet larger numbers in no time.
The Fast Way: Subnetting a Class C Address
When you've chosen a possible subnet mask for your network and need to
determine the number of subnets, valid hosts, and broadcast address that the
mask provides, all you need to do is answer five simple questions:
How many subnets does the chosen subnet mask produce?
How many valid hosts per subnet are available?
What are the valid subnets?
What's the broadcast address of each subnet?
What are the valid hosts in each subnet?
At this point it's important that you both understand and have memorized
your powers of 2. Please refer to the sidebar earlier in this chapter if you need
some help. Here's how you get the answers to those five big questions:
How many subnets? 2
x
- 2 = number of subnets. x is the number of
masked bits, or the 1s. For example, in 11000000, the number of ones
gives us 2
2
- 2 subnets. In this example, there are 2 subnets.
10
111110 = 190
The last valid host
10
111111 = 191
The broadcast address
T A B L E 3 . 6
Subnet 128 (continued)
Subnet
Host
Meaning
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