Subnetting question only gives you the subnet mask to work

Here's the question:

Which of the following addresses can be assigned to a host when using a subnet mask of 255.255.254.0? (select three)

A. 113.10.4.0
B. 186.54.3.0
C. 175.33.3.255
D. 26.35.2.255
E. 152.135.7.0
F. 17.35.36.0

Now the correct answers are B, D, E...Can someone explain to me why? I know when calculating for the CIDR notation that its /23. I just dont understand the steps of how they achieved those answers. Any help would be most appreciated thanks.

Setotek

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suffah

Do you know how to calculate the valid subnet ranges from the subnet mask?

A and F are the actual network, so they can't be assigned to a host. C is the broadcast address for that particular subnet, so it can't be assigned to a host either.

Setotek

If i remember correctly, you basically have to use the high and low order bits. Am I correct?

dynamik

AND the bits of the subnet mask and the address to get the subnet address

Once you have the subnet address, make all the host bits 1 to get the broadcast

A. 113.10.4.0

Subnet Mask

255.255.254.0 - 11111111.11111111.11111110.00000000

IP Address

113.10.4.0

01110001.00001010.00000100.00000000

Subnet Address ---- 113.10.4.0 ---- 01110001.00001010.00000100.00000000

Broadcast Address - 113.10.5.255 - 01110001.00001010.00000101.11111111

Subnet address, so it's not valid.

B. 186.54.3.0

Subnet Mask

255.255.254.0 - 11111111.11111111.11111110.00000000

IP Address

186.54.3.0

10111010.00111000.00000011.00000000

Subnet Address ---- 186.54.2.0

10111010.00111000.00000010.00000000

Broadcast Address - 186.54.3.255 - 01110001.00001010.00000011.11111111

Falls between subnet address and broadcast, so it's valid.

etc.

Hopefully that's right; I'm in a hurry. Gotta go grab another beer and get back to studying

tech-airman

Setotek wrote:

Here's the question:

Which of the following addresses can be assigned to a host when using a subnet mask of 255.255.254.0? (select three)

A. 113.10.4.0
B. 186.54.3.0
C. 175.33.3.255
D. 26.35.2.255
E. 152.135.7.0
F. 17.35.36.0

Now the correct answers are B, D, E...Can someone explain to me why? I know when calculating for the CIDR notation that its /23. I just dont understand the steps of how they achieved those answers. Any help would be most appreciated thanks.

Setotek

Setotek,

For each of the above IP addresses, what class are those IP addreses?

dredlord

If you have them available you should check out the CBTnuggets on subnetting. He teaches this method that is so SIMPLE and works for all types of questions.

Setotek

Tech airman,

A. 113.10.4.0 - Class A
B. 186.54.3.0 - Class B
C. 175.33.3.255 - Class B
D. 26.35.2.255 - Class A
E. 152.135.7.0 - Class B
F. 17.35.36.0 - Class A

And unfortunately, I do not have the CBT nugget for the ccna. All i have are Odom's books.

According to dynamiks, If I can paraphrase what youre saying is that once we convert the subnet mask to binary and perform the Boolean AND, i can figure out if that ip address falls into the range of the Subnet Address and Broadcast Address.

Am I on the right path? Thanks everyone.

Setotek

dynamik

Yea, I think that's the fastest way to do something like this. It's good to know the classes, but I don't think you even need to take things that far with these types of questions.

Check out these for some practice/instruction:
http://www.subnettingquestions.com
http://www.learntosubnet.com/

Glynixx

Hi.

You can get the CBT nugget way of doing subnetting via:
http://nuggetlab.com/

On the left look for CCENT files

Hope this helps!
G.

Glynixx

Glynixx wrote:

Hi.

You can get the CBT nugget way of doing subnetting via:
http://another illegal site.com/

On the left look for CCENT files

Hope this helps!
G.

Mikej412, can you look into this for me? The site above is not illegal, it belongs to cbtnuggets

<confused>
Thanks in advance bro
G.

Setotek

Thanks but the link doesnt work...

tech-airman

Setotek wrote:

Tech airman,

A. 113.10.4.0 - Class A
B. 186.54.3.0 - Class B
C. 175.33.3.255 - Class B
D. 26.35.2.255 - Class A
E. 152.135.7.0 - Class B
F. 17.35.36.0 - Class A

And unfortunately, I do not have the CBT nugget for the ccna. All i have are Odom's books.

According to dynamiks, If I can paraphrase what youre saying is that once we convert the subnet mask to binary and perform the Boolean AND, i can figure out if that ip address falls into the range of the Subnet Address and Broadcast Address.

Am I on the right path? Thanks everyone.

Setotek

Setotek,

For each of the above, what are the default masks?

mamono

Here's the question:

Which of the following addresses can be assigned to a host when using a subnet mask of 255.255.254.0? (select three)

A. 113.10.4.0
B. 186.54.3.0
C. 175.33.3.255
D. 26.35.2.255
E. 152.135.7.0
F. 17.35.36.0

Now the correct answers are B, D, E...Can someone explain to me why? I know when calculating for the CIDR notation that its /23. I just dont understand the steps of how they achieved those answers. Any help would be most appreciated thanks.

Subnet Mask 255.255.254.0 is CIDR /23 which means that the binary form is as follows:

11111111.11111111.11111110.00000000

By involving the 3rd octet, that means it is involved in the subnet range. Think of the 0's are wild cards for the hosts. Anything outside of the 4th octet as a '0' will become a multiplier for the 4th octet value of 256.

Octet Binary to Decimal Chart
128 64 32 16 8 4 2 1 (Converter)

1 1 1 1 1 1 1 0 (3rd Octet in Binary)

The easiest way to determine what the multiplier value will be within the 3rd octet is to look to the left of your 3rd octet '0' values for your 4th octet multiplier.

1 1 1 1 1 1 1 0 (3rd Octet in Binary)
X X X X X X (2) 1 (3rd Octet values that affect 4th Octet)
Let's consider if the CIDR were /23. That would mean 3rd octet binary of 11111110. The value to the left of the '0' is #2. '0' has two values: 0 and 1.

1 1 1 1 1 1 0 0 (3rd Octet in Binary)
X X X X X (4) 2 1 (3rd Octet values that affect 4th Octet)
Let's consider if the CIDR were /22. That would mean 3rd octet binary of 11111100. The value to the left of '00' is #4. '00' has four values: 00, 01, 10, 11.

1 1 1 1 1 0 0 0 (3rd Octet in Binary)
X X X X (8 ) 4 2 1 (3rd Octet values that affect 4th Octet)
Let's consider if the CIDR were /21. That would mean 3rd octet binary of 11111000. The value to the left of '000' is #8. '000' has eight values: 000, 001, 010, 011, 100, 101, 110, 111.

etc etc...

Now that we know that the subnet mask's 3rd octet creates a 4th octet multiplier of 2, we multiple the total number value of the 4th octet by that value. 256 x 2 = 512. This value will include 1 subnet network address, 1 broadcast address, and 510 valid host addresses.

Without knowing the actual IP range, we can only rely on the specifics of subnetting. Each subnet for this type will always begin as follows...

XXX.XXX.0.0 ~ XXX.XXX.1.255
XXX.XXX.2.0 ~ XXX.XXX.3.255
...
XXX.XXX.254.0 ~ XXX.XXX.255.255

We know that every IP address that has a 3rd octet value that is an even decimal value and ends with a 4th octet decimal value of '0' will be the subnet network address.

We know that every IP address that has a 3rd octet value that is an odd decimal value and ends with a 4th octet dedimal value of '255' will be a broadcast address.

All values that are not subnet network addresses or broadcast addresses, are valid host addresses. Just because an IP address has a '0' or a '255' as a 4th octet decimal value, doesn't mean that it is automatically a subnet network address or broadcast address.

A. 113.10.4.0 (Subnet Network: even 3rd, 0 4th)
B. 186.54.3.0
C. 175.33.3.255 (Broadcast: odd 3rd, 255 4th)
D. 26.35.2.255
E. 152.135.7.0
F. 17.35.36.0 (Subnet Network: even 3rd, 0 4th)

B. 186.54.3.0
Within Subnet Range: 186.54.2.0 ~ 182.54.3.255

D. 26.35.2.255
Within Subnet Range: 26.35.2.0 ~ 26.35.3.255

E. 152.135.7.0
Within Subnet Range: 152.135.6.0 ~ 152.135.7.255

I hope that this answers the question.

Setotek

Tech-airman,

The default subnet mask for Class A is 255.0.0.0 and Class B is 255.255.0.0

Mamono,

After studying everyone's inputs I can gather that I can subnet those types of questions. Rather than coverting everything to binary approach, its much faster for me to calculate the ranges by finding the mupltiples of a particular subnet by the given subnet mask.

Thanks everyone. I hope this stuff stays in the my head after my vacation in 2 weeks.