Configure TCP/IP - MCSA Windows Server 2012 R2: Installation and Configuration Study Guide - page 417

CIDR Mask CIDR Mask CIDR Mask

/8 255.0.0.0 /17 255.255.128.0 /25 255.255.255.128

/9 255.128.0.0 /18 255.255.192.0 /26 255.255.255.192

/10 255.192.0.0 /19 255.255.224.0 /27 255.255.255.224

/11 255.224.0.0 /20 255.255.240.0 /28 255.255.255.240

/12 255.240.0.0 /21 255.255.248.0 /29 255.255.255.248

/13 255.248.0.0 /22 255.255.252.0 /30 255.255.255.252

/14 255.252.0.0 /23 255.255.254.0 /31 255.255.255.254

/15 255.254.0.0 /24 255.255.255.0 /32 255.255.255.255

/16 255.255.0.0

Quickly Identifying Subnet Characteristics Using CIDR
Given the limited time you have to dispatch questions in the structured environment of a
Microsoft certification exam, every shortcut to coming up with the correct answer is a plus.
The following method, using CIDR notation, can shave minutes off the time it takes you to
complete a single question. Since you already understand the underlying binary technology
at the heart of subnetting, you can use the following shortcuts, one for each address class,
to come up with the correct answer without working in binary.

Identifying Class C Subnet Characteristics
Consider the host address 192.168.10.50/27. The following steps flesh out the details of the
subnet of which this address is a member:
1.

Obtain the CIDR-notation prefix length for the address by converting the dotted-decimal
mask to CIDR notation.

In this case, /27 corresponds to a mask of 255.255.255.224. Practice converting
between these notations until it becomes second nature.

2.

Using the closest multiple of 8 that is greater than or equal to the prefix length, com-
pute the interesting octet (the octet that increases from one subnet to the next in
increments other than 1 or 0). Divide this multiple by 8. The result is a number corre-
sponding to the octet that is interesting.

In this case, the next multiple of 8 greater than 27 is 32. Dividing 32 by 8 produces the
number 4, pointing to the fourth octet as the interesting one.

3. To compute the incremental value in the interesting octet, subtract the prefix length

from the next higher multiple of 8, which in this case is 32. The result (32 - 27) is
5. Raise 2 to the computed value (2 5 = 32). The result is the incremental value of the
interesting octet.