Class A subnetting Help

[macdiesel]

I feel I can pretty much subnet a Class B or Class C network address with no problems. When it comes to class A I get stumbled because the numbers get very large and I can't do the math quickly in my head.

How would you solve the following problem using the binary method and no shortcuts. I would really appreciate it. I can get the subnet mask no problem. How do i determine what the 303rd subnet would be easily??

Network: 14.0.0.0

Need 500 Subnetworks with as many hosts per subnet as possible
--------------------------------------------------------------------------------
Solve for -

Subnet Mask: ____________________________________

For the 303rd usable subnetwork, give the IP address for:

Wire (network) #:________________________

1st Usable:___________________________

Last Usable:_________________________

Broadcast:______________________________

How many usable subnets are possible? _______________
How many usable hosts/subnet are possible? ________________

[27]

Why the insistence on binary and no shortcuts?

[macdiesel]

Because I want to know why it works the way it does, but if you know shortcuts that will work too. So I can at least know how to jump to say the 303rd subnet quickly. Thanks

[27]

Right.
I need you to write on a sheet of paper.

You need to make three columns.

1. Number of subnets
2. Worth
3. Subnet Mask

Let me know when you've made these three headings.

[macdiesel]

Got it.

[27]

Next, you'll need to make a table with those headings.

Column 1 (Number of subnets).
Start with the number 2 and double it until yo reach 256.
2, 4, 8, 16, 32, 64, 128, 256.

Column 2 (Worth).
Start with 128 and divide by 2 it until you reach 1.
128, 64, 32, 16, 8, 4, 2, 1.

Column 3 (Subnet mask).
The cumulative sum of the previous column.
128, 192, 224, 240, 248, 252, 254, 255.

Let me know when that is written.

[macdiesel]

Ok I have done that.

[27]

You implied that you know what class an address belongs to but I'll cover this as well.

A: 1.0.0.0 to 126.0.0.0
B: 128.0.0.0 to 191.255.0.0
C: 192.0.0.0 to 223.255.255.0

[macdiesel]

Yes so the default subnet mask for 14.0.0.0 is 255.0.0.0. Since I need 500 subnetworks we need to borrow 9 bits (creates 512 usable subnetworks), which makes the new mask 255.255.128.0.

[27]

Ah, but you already have 8 bits.
So you only need to "borrow" one bit to get 9.

Therefore, your new subnet mask should be 255.128.0.0. Do you understand?

[macdiesel]

I thought you didn't count the first octet because it is the default mask. You can only have 1 network with the default mask correct? If you only borrow 1 bit you can't subnet I thought because of the 0 subnet.

[macdiesel]

The website I got the problem from even shows the subnet mask of 255.255.128.0

[27]

I believe you do count the first octet.
If you don't and go for 255.255.128.0, you'll end up with 17 network bits and 15 host bits.
That'll give you far more than 500 subnets.

Online IP Subnet Calculator

[27]

The website I got the problem from even shows the subnet mask of 255.255.128.0

Which site?

[macdiesel]

If I use the calculator you provided and punch in 14.0.0.0 and then 17 mask bits it shows 512 subnets like I stated.

POD #301 Problem 301

[27]

If I use the calculator you provided and punch in 14.0.0.0 and then 17 mask bits it shows 512 subnets like I stated.

POD #301 Problem 301

I think you're right about that.

[macdiesel]

So what is the easiest way to get to the 303rd subnet knowing 14.0.0.0/17

[27]

Right.
So we know that:
subnet 0 is 14.0.0.0
subnet 1 is 14.0.128.0

For each next subnet, you add 128 to the 3rd octet.
The 303rd usable subnet would be subnet 304 (due to subnet 0).

You need a quicker way than writing each one down, I presume?

[macdiesel]

That is correct. That would take for ever to write each subnetwork down.

[27]

Are you doing this to prepare for an exam?