Subnetting Question

tuscani

How many subnetworks and hosts are available per subnet if you apply a /28 mask
to the 210.10.2.0 class C network?

A. 30 networks and 6 hosts.
B. 6 networks and 30 hosts.
C. 8 networks and 32 hosts.
D. 32 networks and 18 hosts.
E. 16 networks and 14 hosts.
F. None of the above

Wouldn't it be 14 hosts and 14 nets assuming no subnet zero? Is the 801 counting subnet zero now?

Alien

16 subnetwork addresses using subnet zero and 14 hosts addresses. This is because the host address of all ones and all zeros is not used. This rule does not affect ubnetwork addresses on the latest IOS and can be applied to older IOS using "ip subnet-zero" command.

Alien

Is the 801 counting subnet zero now?

Atleast they were when i did the exam and that is almost two years ago.

tuscani

hmmm ok.. So I was right.. I just need to know if the latest ver of the exam counts subnet zero.

agustinchernitsky

Well, this is how to make it quick (not sure if this is the best but, works for me):

Learn this scale (1): 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192

And this one (2): 128 192 224 240 248 252 254 255

now, to your question:

/28 would be: 11110000

number of subnets: how many 1s you have? 4? good... what number is in position 4 on scale 1?: 16.

Number of hosts: How many 0s you have? 4 again? good... what number is in position 4 on scale 1?: 16... warning: remember to substract 2 ips... 14.

What is the subnet mask?: 4th place of scale 2: 255.255.255.240

And what is the IP block?: look at scale 1, 4th place: 16 IP blocks (remember, as Alien said, if ip subnet-zero is on, start from 0 always).

This at least works for me... and lets you subnet in your head quite easy.

Also, the scale works for subnetting any class A B or C.

Alien

I just need to know if the latest ver of the exam counts subnet zero.

For the time being atleast untill IPv6 takes root.

tuscani

agustinchernitsky wrote:

Well, this is how to make it quick (not sure if this is the best but, works for me):

Learn this scale (1): 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192

And this one (2): 128 192 224 240 248 252 254 255

now, to your question:

/28 would be: 11110000

number of subnets: how many 1s you have? 4? good... what number is in position 4 on scale 1?: 16.

Number of hosts: How many 0s you have? 4 again? good... what number is in position 4 on scale 1?: 16... warning: remember to substract 2 ips... 14.

What is the subnet mask?: 4th place of scale 2: 255.255.255.240

And what is the IP block?: look at scale 1, 4th place: 16 IP blocks (remember, as Alien said, if ip subnet-zero is on, start from 0 always).

This at least works for me... and lets you subnet in your head quite easy.

Also, the scale works for subnetting any class A B or C.

Thanks.. I know how to subnet.. I just don't know if I should count subnet 0 for the exam.

agustinchernitsky

Sure you do... I just realized I didn't read the last line of your post Silly me!

mp3spy

agustinchernitsky wrote:

Well, this is how to make it quick (not sure if this is the best but, works for me):

Learn this scale (1): 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192

And this one (2): 128 192 224 240 248 252 254 255

now, to your question:

/28 would be: 11110000

number of subnets: how many 1s you have? 4? good... what number is in position 4 on scale 1?: 16.

Number of hosts: How many 0s you have? 4 again? good... what number is in position 4 on scale 1?: 16... warning: remember to substract 2 ips... 14.

What is the subnet mask?: 4th place of scale 2: 255.255.255.240

And what is the IP block?: look at scale 1, 4th place: 16 IP blocks (remember, as Alien said, if ip subnet-zero is on, start from 0 always).

This at least works for me... and lets you subnet in your head quite easy.

Also, the scale works for subnetting any class A B or C.

Maybe I have missed the IP Subnet Zero concept all together...But if that was enabled wouldnt it be 16 hosts/nets and 16 Nets

IP Subnet Zero = net/broadcast address are usable IPs for hosts!?!?!

theseman

Subnet zero only applies to network addresses,

not hosts!!!!

mp3spy

theseman wrote:

Subnet zero only applies to network addresses,

not hosts!!!!

So if IP Subnet Zero is off...you don't use the first and last network address?

theseman

Exactly!! No subnet zero, then your calculation is

(2^n) -2 (can't use a network number of all 0's or 1's)

With subnet zero you can use all of the network addresses, so its just 2^n.

Alien

I'm just curious but aren't you already a CCNA?[/quote]

tuscani

Yes.. I am renewing in a few weeks though. I think it is save to assume subnetting questions do not include subnet zero unless the question says to use it.

david_r

I think it would be safe to assume that both answers won't exist without it being specified in the question.

agustinchernitsky

Well, since Cisco ip subnet-zero is the "new default" on IOS 12.x... I guess when I take the exam I will consider it as a default.

If its a sim... you can always use sh run (if you can) to see if the setting is in place. no?

mp3spy

theseman wrote:

Exactly!! No subnet zero, then your calculation is

(2^n) -2 (can't use a network number of all 0's or 1's)

With subnet zero you can use all of the network addresses, so its just 2^n.

Gotcha!!

J2tehk

agustinchernitsky wrote:

Well, since Cisco ip subnet-zero is the "new default" on IOS 12.x... I guess when I take the exam I will consider it as a default.

If its a sim... you can always use sh run (if you can) to see if the setting is in place. no?

The exam would list everything you need to know when subnetting. Just read the questions carefully. I missed a SIM because I didn't read the entire question. =(

agustinchernitsky

Thanks for the tip J2tehk!