What is Subnetting?

Lessons Discussion
31

Hello Narad

To be honest, this is the first time I’ve come across this, but it is a very interesting scenario!! So why can’t you assign such an IP address and subnet mask combination? Because this will result in a network address of 0.0.0.0, something that is not allowed. Let’s take a look at it in binary:

  • 10.1.1.1 = 00001010.00000001.00000001.00000001
  • 128.0.0.0 = 10000000.00000000.00000000.00000000

To get the network address, you AND them:

00001010.00000001.00000001.00000001
10000000.00000000.00000000.00000000

00000000.00000000.00000000.00000000

The result is 0.0.0.0, which is not an acceptable network address.

You get a clue from the message that a Windows PC will give you if you attempt such an IP address/subnet mask combination:
image

In order for the 128.0.0.0 subnet to be valid, it can only be used with an IP address that has a leading “1” in the binary form of the address. Otherwise, you end up with 0.0.0.0, an invalid network address.

I hope this has been helpful!

Laz