Class A address Subnetting Help

[macdiesel1]

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:_________________________

 

[lerdalt]

Well, I had to cheat first then make certain I could do it. Been a long time since I've had a question like that.

I get 255.255.255.128.0 for the subnet mask.
The only way I know how to find the 303rd usable subnet is to start listing out the start/end IP addresses. I'm going to assume someone else has a better way to find it.

As for the binary math, I'll try to explain, but my first rule when I'm trying to help someone with subnetting is if I start to confuse you...stop reading what I'm saying. My experience has been once you get confused with multiple people trying to explain it, it only makes it all worse.

Ok..first convert 14.0.0.0 to binary=
0000 01110.0000 0000.0000 0000.0000 0000

This is where I might get a little ugly, if you need 500 subnets, determine how many bits you need to represent 500=
500 in binary = 1.1111 0100 = 9 bits

Your subnet mask is broken up like this for a class A:
8 network bits | 24-x subnet/host bits.

The first 8 bits of your mask are already taken for you, the next x bits come from the number of bits to get 500 in binary, so the mask in binary is=
1111 1111.1111 1111.1000 0000.0000 0000

Which become 255.255.255.128.0
Subnet address is 14.0.0.0
First usable address is 14.0.0.1
Last usable address 14.0.127.254
Broadcast address is 14.0.127.255

I'm with you, class A masks are terribly hard because the numbers get so large. And I think everyone will agree real world, you don't run into this, but it's important to understand the binary behind it all.

 

[lerdalt]

BTW, this is one of my favorite sites for checking my work:

http://www.subnetmask.info/

 

[lerdalt]

LOL..ok one more look at this, and I'm not doing this with binary, but taking a shot at the 303rd network.
14.151.128.0?

If I'm right on that one, it's easy to explain, but I probably used a shortcut.

 

[macdiesel1]

You are correct. That is the 303rd network. How did you get to that? I will take shortcuts at this point. Thank you

 

[lerdalt]

well...the subnet mask was coming out so ranges were going to be:
14.0.0.0 - 14.0.127.255
14.0.128.0 - 14.0.255.255
14.1.0.0 - 14.1.127.255
14.1.128.0 - 14.1.255.255

basically for each 14.x address there were going to be 2 subnets. 300/2 = 150
so...the 300th subnet was going to be 14.150.0.0
301 = 14.150.128.0
302 = 14.151.0.0
303 = 14.150.128.0

 

[burtsbees]

That's an extreme example. You'll never see a question like that on the Cisco exam. Usually you'll see subnet addresses repeat every nth time, like a /28 every 16, so like the 7th subnet would be 16X7, which is .112 That's more like a Cisco CCNA question...

Burt

 

[lerdalt]

Also..while it's good to understand the binary behind it, you will want to learn the short cuts for the exams.

 

[CiscoGuy33]

macdiesel1,

Burt is right, good to practice, practice and do more practice on subnetting and YOU will see subnetting in some way, shape or form in SEVERAL questions on Cisco exams, but all I have always had were straight forward.

Most tended to deal with Class B or Class C, I think on my White Board - before I started the exam I extended my .240 and .248
eg -
0, 8, 16, 24, 32, 40, 48, 56, 64, 72 ....
0, 16, 32, 48, 64, 80, 96, 112, 128, 144.....

This way if they asked me about 192.168.13.137 with a mask of 255.255.255.240 I would know the subnet was 192.168.13.128 and the broadcast was 192.168.13.143 etc.

This always helped if they asked me if an address was a node, a subnet or a broadcast address or use 2 addresses from the 4th subnet for the serial interface assuming a mask of 255.255.255.248.....

Hope this helps!

E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[burtsbees]

Burt is right"

Sky is blue, grass is green...

Burt

 

[Cluebird]

OK, I can't stand this...Be careful with the blocks, subnet zero is the first subnet so with current rules you can't just say "the 7th subnet would be 16x7..." For the /28 the correct answer for the 7th subnet is .96.

As for the class A, the mask lerdalt means is 255.255.128.0 or /17. (I think the keyboard entered an extra octet?) The 303 subnet is actually 14.151.0.0 because of subnet zero being the first subnet.

Many of the subnet practices don't allow subnet zero, however classless addressing and subnet zero have been Cisco defaults in IOS since 12.0 came out around 2000.

HTH

 

[CiscoGuy33]

Cluebird,

You are right - understand if they are or are not using subnet zero.

In every Cisco exam I had, they gave some sort of hint or just said the router is running using Cisco defaults, but normally is was very clear, but sometimes you might have to understand a part of a "show run" that had "no subnet zero" listed

E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[burtsbees]

Wrong-o, my friend...

.0
.16
.32
.64
.80
.96
.112

the seventh subnet is the seventh subnet---7 times the multiple...
Gene and Tad...what can I say?

Burt

 

[lerdalt]

Thanks Cluebird, I hate hate it when my keykeyboard gets stuck like that.

 

[CiscoGuy33]

Burt,

I think the point - at least I was trying to make is to read the question because it might say something like the 7th "useable" subnet or something to tell you NOT to use subnet zero -
.0 not used
.16 1
.32 2
.64 3
.80 4
.96 5
.112 6
.128 7

I had a question like this on an exam or a practice exam that wanted the x subnet but it also gave you part of the "show Run" and from that you had to understand that they were not using subnet zero. It must have been a practice exam because I remember it detailed the answer and showed you what to look for in the question!!

The key is to read the question and ALL of the answers, Cisco LOVES to ask a tricky question and have what appears to be the right answer (with a quick read) as 1 or 2 but the real answer is answer 3 or 4 - many people jumped on what looked like the right answer but Cisco ALWAYS wants the BEST right answer. Cisco did this all the time in the Network Academy tests to help train the students to read and pick the best of 2 "right" answers!!

E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[Cluebird]

Buirt,

Try this:
0
16
32
48
64
80
96

You skipped a block between 32 and 64.

 

[burtsbees]

Where's my medication...

Burt

 

[macdiesel1]

Thanks everyone for the tips!

 

[burtsbees]

Come again, mac...lol

Burt

 

[DOLLAR61]

I'm trying to get 500 subnets for 16.0.0.0/8. Can someone help me. I'm just not getting it.