152
Chapter 3
Internet Protocols
How many hosts per subnet? We have 6 host bits off (11000000), so
the equation would be 2
6
- 2 = 62 hosts.
What are the valid subnets? 256
- 192 = 64, which is the first subnet
and our base number or variable. Keep adding the variable to itself
until you reach the subnet mask. 64
+ 64 = 128. 128 + 64 = 192, which
is invalid because it is the subnet mask (all subnet bits turned on). Our
two valid subnets are, then, 64 and 128.
What's the broadcast address for each subnet? The number right
before the next subnet is all host bits turned on and equals the broad-
cast address.
What are the valid hosts? These are the numbers between the sub-
nets. The easiest way to find the hosts is to write out the subnet
address and the broadcast address. This way the valid hosts are obvi-
ous. The following table shows the 64 and 128 subnets, the valid
host ranges of each, and the broadcast address of both subnets.
See? We really did come up with the same answers as when we did it the
binary way, and this way is so much easier because you never have to do
any binary-to-decimal conversions! About now, you might be thinking that
it's not easier than the first method I showed you. And I'll admit, for the
first subnet with only two subnet bits--you're right, it isn't that much
easier. But remember, we're going after the gold: being able to subnet in
your head. And to do that, you need one thing: practice!
Practice Example #2C: 255.255.255.224 (/27)
This time, we'll subnet the network address 192.168.10.0 and subnet mask
255.255.255.224.
192.168.10.0
= Network address
255.255.255.224
= Subnet mask
How many subnets? 224 is 11100000, so our equation would be
2
3
- 2 = 6.
The subnets (do this first)
64
128
Our first host (perform host addressing last)
65
129
Our last host
126
190
The broadcast address (do this second)
127
191
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