Information Technology Reference
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4.
Recall the value of the interesting octet from the original address (50 in this case).
Starting with 0, increment by the incremental value until the value is exceeded. The
values then are 0, 32, 64, and so on.
5.
The subnet in question extends from the increment that is immediately less than or
equal to the address's interesting octet value to the address immediately before the next
increment. In this example, 192.168.10.50/27 belongs to the subnet 192.168.10.32,
and this subnet extends to the address immediately preceding 192.168.10.64, which is
its broadcast address, 192.168.10.63.
Note that if the interesting octet is not the fourth octet, all octets after the interesting
octet must be set to 0 for the subnet address.
6.
The usable range of addresses for the subnet in question extends from one higher
than the subnet address to one less than the broadcast address, making the range
for the subnet in question 192.168.10.33 through 192.168.10.62. As you can see,
192.168.10.50/27 definitely falls within the subnet 192.168.10.32/27.
Identifying Class B Subnet Characteristics
Using the steps in the previous section, find the subnet in which the address 172.16.76.12
with a mask of 255.255.240.0 belongs.
1.
The corresponding CIDR notation prefix length is /20.
2.
The next multiple of 8 that is greater than 20 is 24. 24
×
8 = 3. Octet 3 is interesting.
24 - 20 = 4, so the incremental value is 2
4
= 16.
3.
4.
The increments in the third octet are 0, 16, 32, 48, 64, 80, and so on.
5.
The increments of 64 and 80 bracket the address's third-octet value of 76, making the
subnet in question 172.16.64.0, after setting all octets after the interesting octet to 0.
This subnet's broadcast address is 172.16.79.255, which comes right before the next
subnet address of 172.16.80.0.
6.
The usable address range then extends from 172.16.64.1 through 172.16.79.254.
Identifying Class A Subnet Characteristics
Try it one more time with 10.6.127.255/14. Combine some of the related steps if possible:
1.
The prefix length is 14. The next multiple of 8 that is greater than or equal to 14 is 16.
16
×
8 = 2, so the second octet is interesting.
16 - 14 = 2, so the incremental value in the second octet is 2
2
= 4.
2.
3.
The corresponding second-octet value of 6 in the address falls between the 4 and 8
increments. This means that the subnet in question is 10.4.0.0 (setting octets after the
second one to 0) and its broadcast address is 10.7.255.255.
4.
The usable address range is from 10.4.0.1 through 10.7.255.254.
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