CCNA Cisco Certified Network Associate Study Guide - FTP Server

Subnetting 137
In a Class C address, only 8 bits is available for defining the hosts.
Remember that subnet bits start at the left and go to the right, without skipping
bits. This means that subnet masks can be
10000000=128
11000000=192
11100000=224
11110000=240
11111000=248
11111100=252
11111110=254
Now, the RFCs state that you cannot have only one bit for subnetting,
since that would mean that the bit would always be either off or on, which
would be illegal. So, the first subnet mask you can legally use is 192, and the
last one is 252, since you need at least two bits for defining hosts.
The Binary Method: Subnetting a Class C Address
In this section you will learn how to subnet a Class C address using the
binary method. We will take the first subnet mask available with a Class C
address, which borrows two bits from subnetting. For this example, we are
using 255.255.255.192.
192=11000000 Two bits for subnetting, 6 bits for defining the hosts in
each subnet. What are the subnets? Since the subnet bits can’t be both off
or on at the same time, the only two valid subnets are
or
� 01000000=64 (all host bits off)
� 10000000=128 (all host bits off)
The valid hosts would be defined as the numbers between the subnets,
minus the all host bits off and all host bits on.
To find the hosts, first find your subnet by turning all the host bits off,
then turn all the host bits on to find your broadcast address for the subnet.
The valid hosts must be between those two numbers. Table 3.4 shows the 64
subnet, valid host range, and broadcast address.