Why is an invalid broadcast address?

I’m not sure if this is the right place to post this.
I’ve been following the CCENT official certification book(100-105) and came upon this question in the “do I know this already?” quiz. The books only covered /24 subnetting only so far.

  1. Which of the following is a network broadcast address?

As no subnetting notation has been included, I’ll stick with .255 ending as being broadcast.
a = seems correct.
b = incorrect. ends with .1
c = incorrect, it’s a class d multicast.
d = seems corrects too.

The answer says ONLY D is correct.

So why is A incorrect? My understanding: as the network ID to as valid IP addresses. as the broadcast

Class a = 8 Bits network ID. 24 Bits Host ID. (0 subnet bits as theirs only 1 whole subnet? is this correct?)
subnet mask =

Both A and D are correct from my understanding. There is practically no difference between those Answers.

Hi Bilal,
A) is indeed incorrect. Because you stated there is no subnetting in place.
10 is inside range from 1 to 127 making it class A. 10 is the major network network here.

Your explanation is correct. Default class A network is /8, making it default subnet mask. If you want to get broadcast address simply turn all host bits to 1, thus you will end with as broadcast address of major network 10. This is really huge broadcast address.
What is important for your question:

  • Class A has default subnet mask
  • Class B has default subnet mask
  • Class C has default subnet mask

If you want to get broadcast address for these default major networks you have to turn octects with 0 to 255.

1 Like

Hi Michal,
Thanks for the great explanation, really appreciate it! :grin:
After reading it multiple times, I’ve finally understood the concept! :smiley: