000000 = 192
The subnet address
000001 = 193
The first valid host
111110 = 254
The last valid host
111111 = 255
The broadcast address
IP Subnetting and Variable Length Subnet Masks (VLSMs)
Hopefully, you understood what I was trying to show you. The example I presented only used 2 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.
In the following section, I’m going to teach you an alternate method of subnetting that makes it easier to subnet larger numbers in no time.
Since the 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 chapter, is vital. Using the binary method can take you way too long and you could fail the exam even if you know the material!
The Fast Way: Subnetting a Class C Address
When you’ve chosen a possible subnet mask for your network and need to determine the num- ber of subnets, valid hosts, and broadcast addresses of a subnet 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? 2x = number of subnets. x is the number of masked bits, or the 1s. For example, in 11000000, the number of ones gives us 22 subnets. In this example, there are 4 subnets.