VLSM is driving me absolutely out of my mind....please help if you have a moment...

I don't have anymore hair to pull out of my head at this point but here we go....

The problem in the book is asking me to find the subnet ID and Subnet broadcast address for 10.1.1.1 /20 (255.255.240.0)

and I know exactly what to do up until getting the broadcast address.

I know my Subnet ID is: 10.1.0.0

Then it goes up in increments of 16 (the magic number 256-240=16), so, 10.1.16.0, 10.1.32.0, 10.1.48.0 and so on....until the next subnet block of address that start with 10.1.1.0, 10.1.1.16, 10.1.1.32, 10.1.1.48

Now here is where I get pissed off....the book says the Broadcast address is 10.1.15.255, but how the hell does it know to stop at that specific address?

I'm so frustrated and lost....and I have nowhere else to ask for help....would appreciate any and all help, my exam date is closing in on me and I feel like playing in traffic...

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CiscoWayne

Right the mask in binary:

11111111.11111111.11110000.00000000

The third octet is the ‘interesting octet’

So your ranges are

10.1.0.0 – 10.1.15.255
10.1.16.0 – 10.1.31.255
10.1.32.0 – 10.1.47.255

Etc etc

The ip address falls into the first subnet. So the network address is 10.1.0.0 and the broadcast is 10.1.15.255

I'm just learning subnetting myself, going through the todd lammle book. So please, someone correct me if I'm wrong.

_Gonzalo_

CiscoWayne wrote: »

I'm just learning subnetting myself, going through the todd lammle book. So please, someone correct me if I'm wrong.

You´re absolutely correct.

CiscoWayne

_Gonzalo_ wrote: »

You´re absolutely correct.

Sweet, thanks mate.

quickman007

The broadcast address is the last address in the range. The subnet ID is 10.1.0.0. What's the next subnet ID? 10.1.16.0, so the last address in the 10.1.0.0 range is 10.1.15.255.

digitheads

you think that's fun - that one is really simple - on the exam they mix the subnet masks within the same ip scope... subnet one will be a block of 8, subnet two a block of 32, then subnet 3 a block of 64.... you have to be able to calculate network ip's and broadcast ip's for each subnet...

Alceo

Only remember to go from the largest block to the smallest one and you should not have any problem.

_Gonzalo_

digitheads wrote: »

you think that's fun - that one is really simple - on the exam they mix the subnet masks within the same ip scope... subnet one will be a block of 8, subnet two a block of 32, then subnet 3 a block of 64.... you have to be able to calculate network ip's and broadcast ip's for each subnet...

Well, if you progress in networking, you will end up doing subnetting with no pen or paper... So you´d better start now!

Seriously, if you want to pass CCNA and also be able to deliver at work, you need to understand things. Using binary almost as well as decimal is a consequence of this.

Nans

Then it goes up in increments of 16 (the magic number 256-240=16), so, 10.1.16.0, 10.1.32.0, 10.1.48.0 and so on....until the next subnet block of address that start with 10.1.1.0, 10.1.1.16, 10.1.1.32, 10.1.1.48

I think you got it wrong the subnet blocks starts in the second octet since the mask is 255.255.240.0 you started sunetting from 3rd active octet and then you move on to the octets on the left which are netwok bits so once the active block is over i.e 10.1.0.0 it moves to 10.2.0.0 not 10.1.1.1 ( this is a host address of the 1st subnet you were speaking).

So it will be like this

1st set of subnet---10.1.0.0-10.1.255.255,10.1.16.0-10.1.31.255,10.1.32.0-10.1.47.255.......10.1.240.0-10.1.255.255(Subnet Id's-Broadcast Id's)
2nd set of subnet---10.2.0.0-10.2.255.255,10.2.16.0-10.2.31.255,10.2.32.0-10.2.47.255.......10.2.240.0-10.2.255.255(Subnet Id's-Broadcast Id's)
the last set -10.255.0.0-10.255.255.255,10.255.16.0-10.255.31.255,10.255.32.0-10.255.47.255.......10.255.240.0-10.255.255.255(Subnet Id's-Broadcast Id's)

As some one already said Todd Lammle explained it well enough.

My personal opinion is try to keep your calm I know its tough and I am also going thorough the same so what i wanted to say is dont pluck you hair brother

lol you can rewrite certificate you cant grow you hair back

Experts please correct me if i am wrong

ccie14023

As CiscoWayne said, write it in binary. I had an instructor a long time ago who said "the only way to do subnetting is in binary."

1. Write the address in binary, being careful to keep the octets separate.
2. Count out the mask (20 bits) starting from the left and draw a line.
3. Everything to the left of the line is the "network" portion of the address and anything to the right is the "host" portion.
4. If you want the network address, zero out everything to the right of the line and convert the result back to decimal.
5. If you want the broadcast address, make everything to the right of the line 1's and convert back to decimal.

Pheonyx

thank you guys so much! I totally got it now. just a matter of practicing this along with all the other subnetting stuff. thanks again!! very much appreciated!

JoJoCal19

As a follow-up question to this scenario, given 10.1.0.0/20, is there a total of 4096 IP addresses available on the 10.1 network space?