Subnetting problem

I'm not sure if this a typo from my professor but when I calculate the problem below I get the following:

Subnet Mask: 255.255.255.128
Subnet Address: 128.1.2.0
First Usable: 128.1.2.1
Last Usable 128.1.2.126
Broadcast: 128.1.2.127

I don't get why his Subnet address is 128.1.1.0 and what does it mean by calculate for the "2nd" subnet. I don't know what to look for when it asking for a x number subnet. Thank you

This is from my professor, is this correct?

Subn.jpg

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no!all!

Using a 255.255.255.128 mask only gives you two subnets: 128.1.2.0 and 128.1.2.128, the 2nd subnet would be 128.1.2.128, hope this helps.

myke2424

Thank a lot, but how come on the second subnet you wrote the 3rd octet changes from a 2 to 1? I thought you don't change that because the 3rd octet in the subnet mask is 255.

NetworkNewb

In the attached picture if you put 2s instead of 1s in the 3rd octet of those answers, they will all be correct...

Just a typo by him I'm guessing

NetworkNewb

no!all! wrote: »

Using a 255.255.255.128 mask only gives you two subnets: 128.1.2.0 and 128.1.1.128, the 2nd subnet would be 128.1.2.128, hope this helps.

if you use that subnet on a class B network it will work. I blame the professor for not being clear on that though.

myke2424

So both answers would be considered correct?

NetworkNewb

No it is a 128, it should be a Class B. Didn't really think about it... so your answers look good.

myke2424

Apparently his is correct according to him how is that even possible.

NetworkNewb

myke2424 wrote: »

Apparently his is correct according to him how is that even possible.

He is just stubborn I guess

Just plug ip address and subnet mask into here Online IP Subnet Calculator

DLSK12

You are correct.

Deathmage

Use Binary, it helps explain it. - here i'll show you.

Subnet Mask: 255.255.255.128
Subnet Address: 128.1.2.0
First Usable: 128.1.2.1
Last Usable 128.1.2.126
Broadcast: 128.1.2.127

1st off you need to know this:

Class A: 1 - 126
Class B: 128 - 191
Class C: 192 - 224
(127 is reserved for loopback)

Look at this picture, I'll do a problem from my workbook that I'm using for subnetting currently.

I'm doing this one for 172.100.5.0 with a mask like yours of 255.255.255.128

So since we know the 4th octet is the interesting one, we focus there. And now by looking at the subnets (which go by 2's from left to right) and the hosts (wihch go by 2's from right to left) you can see with your interesting number of 128 in the picture above you basically just draw a imaginary line between the Binary value of 128 and 64. So this leave you with 128 addresses per subnet with 512 subnets.

So....

This means you have 128 addresses per subnet range but there is a catch you need one for the network ID and one for the Broadcast ID, so anytime you have a host range always minus two. So in this case your Network ID would be 172.100.5.0 and your broadcast ID would be 172.100.5.127 and your usable IP range would be what in-between so that's 172.100.5.1 to 172.100.5.126.

Now since you know there is 256 bits in one octet you can minus 128 from 256 and still have 128 left over so you can have two ranges of 128 addresses.

The 2nd range would be a Network ID of 172.100.5.128 and a range of 172.100.5.129 to 172.100.5.255 and a Broadcast ID of 172.100.5.256.

Anytime you need to understanding subnetting just use binary and use the graph above as a reference. It's helped me understand subnetting. See I've start at the root with binary, some people use the power of 2's but that should only be done as a shortcut after you understand binary fully.

One thing I recommend is practise a little bit each day, subnetting is one of those things until you master it that if you don't use it you forget it. 5 to 10 minutes each days does wonders.

Hope this helps!!!!

subnetting problem.JPG

Jon_Cisco

The key to your problem is in condition f.
This is probably an example from an old book that does not allow you to use subnet 0.

So
128.1.1.0 (in the past the 0 subnet was skipped)
128.1.1.128 (first usable)
128.1.2.0 (second usable)
128.1.2.128 (third usable)

jvrlopez

Tell your teacher he is wrong and you will now be teaching the class.