ICND1 subnetting question that i'm not completely understanding, HELP? :)

Beany

Morning All, my first post and hello to everyone.

Q1, In the book, ICND1 Official Exam Certification Guide by Wendell Odom (2nd edition), i'm struggling to grasp the understanding of the following question in Chapter 12 (question :

Which of the following subnet masks let a Class A network have up to 150 hosts per subnet and supports 164 subnets?

A. 255.0.0.0
B. 255.255.0.0
C. 255.255.255.0
D. 255.255.192.0
E. 255.255.252.0
F. 255.255.255.192

The answer in the book is: B,C,D and E. Is this answer correct and not a mistake in the book? If it's correct can someone please explain to me how we end up with this answer? I've been following LORDFLASHEART examples on this website and it would easier for me to understand it if it can be explained by someone using the same technique?? (i just found LORDFLASHEART way of subnetting the easiest)..


Q2, for subnetting practice, im doing the following:

CBT nuggets subnetting videos
Book
Subnettingquestions.com

Will this be enough for me to cover all subnetting needs in ICND1?

All reply will be appreciated. Thanks in advance.

B.

NetworkVeteran

Which of the following subnet masks let a Class A network have up to 150 hosts per subnet and supports 164 subnets?

Fact #1: The network mask for a class A network is 255.0.0.0.
Fact #2: 2^8 = 256
Fact #3: 2^16 = 65535

A) 255.0.0.0 - This should be immediately ruled out as silly. Zero subnet bits = Zero subnets!!! This does NOT meet the requirements.
255.255.0.0 - So, 8 subnet bits (256 subnets) and 16 host bits (65,535-2 hosts per subnet). This meets the requirements.
C) 255.255.255.0 - So, 16 subnet bits (65,535 subnets) and 8 host bits (256-2 hosts). This meets the requirements.
D) 255.255.192.0 - We don't need to calculate this out what "192" means. Laziness is a virtue! Clearly, there are at least 8 subnet bits (256+ subnets) and at least 8 host bits (256+ -2 hosts). This meets the requirements.
E) 255.255.252.0 - Again, at least 8 subnet bits and at least 8 host bits. No calculation. This meets the requirements.
F) 255.255.255.192 - Now we have to be careful. We have at least 16 subnet bits (65,535+ subnets) but do we have enough hosts? A .192 means 11000000 in the last octet. In other words, we have 6 host bits. 2^6-2 = 62 hosts. This does NOT meet the requirements.

Answer: B,C,D,E

I have no idea what the LORDFLASHHEART method is, but if you want to be speedy at subnetting, it's helpful to know when you don't have to bother calculating anything.

mapletune

A.

Class A default network mask 255.0.0.0
thus, with a subnet of 255.0.0.0 you only get 1 network, the default network.

B.

255.255.0.0 borrows 8 host bits from 255.0.0.0 to form 256 subnets
thus, you get 256 subnets and 65,534 hosts

C.

255.255.255.0 borrows 16 host bits from 255.0.0.0 to form 65,536 subnets
thus, you get 65,536 subnets and 255 hosts

D.

255.255.192.0 borrows 10 host bits from 255.0.0.0 to form 1024 subnets
thus, you get 1024 subnets and 16,382 hosts

E.

255.255.255.252.0 borrows 14 host bits from 255.0.0.0 to form 16,384 subnets
thus, you get 16,384 subnets and 1022 hosts

F.

255.255.255.192 borrows 18 host bits from 255.0.0.0 to form 262,144 subnets
thus, you only have 6 host bits left. 2^6 - 2 = 62 hosts.


[Edit]
i guess i type really slow =p hehe

[Edit2]
Sure, if you can nail all subnettingquestions.com problems in 30 seconds or less, that means you are way prepared for the ICND1 exam.

fsanyee

Thats easy. 1st: you find the subnet mask for 164 subnet, which is 255.255.0.0 2nd: you find the mask for 150 host: 255.255.255.0 everything between thos two will work.

NetworkVeteran

fsanyee wrote: »

Thats easy. 1st: you find the subnet mask for 164 subnet, which is 255.255.0.0 2nd: you find the mask for 150 host: 255.255.255.0 everything between thos two will work.

Even better! Do as little math as possible when solving subnetting questions. You will be faster, and less likely to make an accidental calculation error.

Beany

quality, makes sense now...

many thanks