gthorsen
09-18-2008, 09:54 PM
This is one of the questions from the sybex test engine that came with Todd Lammle's CCNA study guide. In the answer to the question it tells you that you use 2 to the n bits to find the number of available subnets. However, in "CCNA Flash Cards and Exam Practice Pack Third Edition" it sais you use the 2 to the n - 2 formula for determining the available amount of subnets. In the question from the practice exam below the answer is different depending on which formula you use. Who is right? My understanding was that 2 to the n - 2 was for determining the amount of hosts where -2 was subtracted because 1 host is reserved as a broadcast address and the other is the classful network address.
Question in the practice exam
Your Internet service provider has assigned your organization a Class C address of 204.16.120.0. Your organization requires that you divide the Class C address into eight different networks to support your campus networking environment. What subnet mask could you use that would make the best use of the available addresses?
A> 255.255.255.224 [correct answer according to the Sybex test engine's answer based on 2 to the nth which equals 8 available subnets]
[incorrect based on "CCNA Flash Cards and Exam Practice Pack Third Edition" whose formula would give you only 6 available subnets]
B> 255.255.255.240 [correct answer according to "CCNA Flash Cards and Exam Practice Pack Third Edition" based on 2 to the nth -2 which is 16 available subnets]
C> 255.255.255.248
D> 255.255.255.252
Answ:
Use the formula 2 to the n power to determine the number of valid subnets that can be created when borrowing bits from the host portion of an IP address. Borrowing 3 bits (2 to the third power) creates eight valid subnets, and a minimum of eight subnetworks are required.
Question in the practice exam
Your Internet service provider has assigned your organization a Class C address of 204.16.120.0. Your organization requires that you divide the Class C address into eight different networks to support your campus networking environment. What subnet mask could you use that would make the best use of the available addresses?
A> 255.255.255.224 [correct answer according to the Sybex test engine's answer based on 2 to the nth which equals 8 available subnets]
[incorrect based on "CCNA Flash Cards and Exam Practice Pack Third Edition" whose formula would give you only 6 available subnets]
B> 255.255.255.240 [correct answer according to "CCNA Flash Cards and Exam Practice Pack Third Edition" based on 2 to the nth -2 which is 16 available subnets]
C> 255.255.255.248
D> 255.255.255.252
Answ:
Use the formula 2 to the n power to determine the number of valid subnets that can be created when borrowing bits from the host portion of an IP address. Borrowing 3 bits (2 to the third power) creates eight valid subnets, and a minimum of eight subnetworks are required.