CCNA Cisco Certified Network Associate Study Guide - FTP Server

Subnetting 155
3. 256–240=16. However, since the second octet is 255, or all subnet bits
on, we can start the third octet with 0 as long as a subnet bit is turned
on in the second octet. So the subnets become 10.1.0.0, 10.1.16.0,
10.1.32.0, and 10.1.48.0, all the way to 10.1.240.0. The next set of
subnets would be 10.2.0.0, 10.2.16.0, 10.2.32.0, 10.2.48.0, all the
way to 10.2.240.0. Notice that we can use 240 in the third octet as
long as all the subnet bits in the second octet are not on. In other
words, 10.255.240.0 is invalid because all subnet bits are turned on.
The last valid subnet would be 10.255.224.0.
4. First find the broadcast addresses in step 5, then come back and perform
step 4 by filling in the host addresses.
5. Find the broadcast address of each subnet, which is always the number
right before the next subnet.
Table 3.9 shows some examples of the host ranges.
TABLE 3.9 Valid Host Ranges for a Class A 255.255.240.0 Mask
Practice Example 3: 255.255.255.192
Let’s do one more example using the second, third, and fourth octets for subnetting.
1. 2 18 –2=262,142 subnets.
2. 2 6 –2=62 hosts.
First Subnet Second Subnet Last Subnet
Subnet 10.1.0.0 10.1.16.0 10.255.224.0
First Host 10.1.0.1 10.1.16.1 10.255.224.1
Last Host 10.1.15.254 10.1.31.254 10.255.239.254
Broadcast 10.1.15.255 10.1.31.255 10.255.239.255
3. Now, we need to add subnet numbers from the second, third, and
fourth octets. In the second and third, they can range from 1 to 255,
as long as all subnet bits in the second, third, and fourth octets are not