Subnetting
151
How many hosts per subnet? 2
x
- 2 = number of hosts per subnet.
x is the number of unmasked bits, or the 0s. For example, in 11000000,
the number of zeros gives us 2
6
- 2 hosts. In this example, there are
62 hosts per subnet.
What are the valid subnets? 256
- subnet mask = block size, or base
number. For example, 256
- 192 = 64. 64 is the first subnet. The
next subnet would be the base number itself, or 64
+ 64 = 128, (the
second subnet). You keep adding the base number to itself until you
reach the value of the subnet mask, which is not a valid subnet
because all subnet bits would be turned on (1s).
What's the broadcast address for each subnet? The broadcast address
is all host bits turned on, which is the number immediately preceding
the next subnet.
What are the valid hosts? Valid hosts are the numbers between the
subnets, minus all 0s and all 1s.
I know this can truly seem confusing. But it really isn't as hard as it seems
to be at first--just hang in there! Why not try a few and see for yourself?
Subnetting Practice Examples: Class C Addresses
Here's your opportunity to practice subnetting Class C addresses using the
method I just described. We're going to start with the first Class C subnet mask
and work through every subnet that we can using a Class C address. When
we're done, I'll show you how easy this is with Class A and B networks too!
Practice Example #1C: 255.255.255.192 (/26)
Let's use the Class C subnet mask from the preceding example,
255.255.255.192, to see how much simpler this method is than writing out the
binary numbers. We're going to subnet the network address 192.168.10.0 and
subnet mask 255.255.255.192.
192.168.10.0
= Network address
255.255.255.192
= Subnet mask
Now, let's answer the big five:
How many subnets? Since 192 is two bits on (11000000), the answer
would be 2
2
- 2 = 2. (The minus 2 is the subnet bits all on or all off,
which are not valid by default.)
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