I’ve just received an email asking for help.

Would you please clarify this concept for me on how to find the zero subnet and broadcast address of a given IP address.

For Example: The given IP address was 200.1.2.0/28

My Working:

255.255.255.240
200.  1.  2.  0
200.  1.  2.  0 Zero Subnet
200.  1.  2. 16
200.  1.  2. 32
200.  1.  2. 48
...
...
200.  1.  2.240

Is 200.1.2.240 the Subnet Broadcast? Or 200.255.255.240?

My reply:

Alfred,

You have exactly the correct answer (200.1.2.240)… but you have asked the wrong question! And your confusion is understandable.

Firstly there are two concepts

The subnet broadcast is the address that can be used to send information to all hosts on that subnet.

The broadcast subnet is the last subnet that can be assigned for a given subnet mask, just as the zero subnet is the first subnet that can be assigned for a given mask.

Given the context of your question, I think you meant to ask

Is 200.1.2.240 the Broadcast Subnet? Or 200.255.255.240?

To answer this question, you have to ask yourself “Am I dealing with a class A, class B or class C address?”

Since you are dealing with a class C address, the first three octets are never going to change, so the 200.1.2.240 answer is correct.

If however, you were dealing with a class B address, like 150.1.2.0, only the first two octets would stay fixed, so the working (using the 255.255.255.240 mask again) would be:

255.255.255.240
150.  1.  2.  0 (Zero subnet)
150.  1.  2. 16
150.  1.  2. 32
...
150.  1.  2.240
150.  1.  3.  0
150.  1.  3. 16
...
150.  1.255.240 (Broadcast subnet)

Which would make 150.1.255.240 the broadcast Subnet.

The alternate answer you gave (200.255.255.240) is more like what you would see for a class A address, like 100.1.2.0, because with class A addresses, only the first octet is fixed.

255.255.255.240
100.  1.  2.  0 (Zero subnet)
100.  1.  2. 16
100.  1.  2. 32
...
100.  1.  2.240
100.  1.  3.  0
100.  1.  3. 16
...
100.255.255.240 (Broadcast subnet)