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.

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 =
I believe it's my lack of understanding of how subnet masks correlate to Ip addresses

  • 33
    The correct answer to the certification question would be: "This is a stupid question. Classes are long dead." Or, to be more polite: "It is impossible to tell without knowing the network size." Or, a counter-question: "Why are testing for decades-obsolete technology, I want my money back!" Commented Oct 10, 2019 at 17:55
  • 2
    "As no subnetting notation has been included". This question has nothing to do with subnetting. It would be a valid question even if no subnetting existed. This questino is entirely about network addresses and network broadcast addresses. It has nothing to do with subnets. VLSM predated CIDR and they are two different things. "I believe it's my lack of understanding of how subnet masks correlate to Ip addresses". Nope. Question is not about subnets, it's about networks. A subnet is a portion of a network. Commented Oct 10, 2019 at 20:46
  • 8
    This question has been made obsolete in the last millennium: Classes are long dead and IPv4 is obsolete. Commented Oct 10, 2019 at 21:08
  • 8
    @DavidSchwartz You missed your coffee today. It's been decades since a network could be defined without its prefix length. Class A, B or C networks are no longer a thing.
    – jcaron
    Commented Oct 10, 2019 at 23:19
  • 1
    @jcaron I agree. But that has nothing to do with subnets or subnetworking. VLSM and CIDR are two different things. Commented Oct 11, 2019 at 15:30

4 Answers 4


I believe the book wrongly assumes network classes are still in effect. So a) would be a "Class A" network, where would be the broadcast address. Another hint: There is no explicit network size specified (/24, /27, ..) so it is implied you know about network classes. Classical example of outdated literature.

  • 10
    This question is indeed incomplete without stating the given subnet size resp. subnet mask length. Even could be considered the broadcast address of (but then again.. maybe not, since /31 are a bit peculiar in that context). Commented Oct 10, 2019 at 9:13
  • 1
    Thanks for the great answer :) I've just found out that because the default subnet mask of class A is, then the broadcast can only be you've specified) Commented Oct 10, 2019 at 9:16
  • 2
    @Marc'netztier'Luethi /31 subnets have no broadcast address (no use for p2p), so can't be one, regardless of prefix length. ;-)
    – Zac67
    Commented Oct 10, 2019 at 9:25
  • 6
    @DavidSchwartz How exactly are "networks" and "subnets" different, ever since CIDR? Commented Oct 10, 2019 at 20:55
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    "Classical example of outdated literature". I see what you did there. Commented Oct 12, 2019 at 11:20

This is a "traditional" exam. question which contains:

  1. Missing information
  2. A trap
  3. A hint

The missing information is the subnet mask or CIDR number of bits.

The trap is answer (a) : is a broadcast address, as is, but without mask information, we must assume classful addressing, and is a unicast address.

The hint is the word network in the question. Being pedantic, is a network and is a subnet.

Those who voted for Sebastian Wiesinger's answer would be marked wrong in the exam.

This sort of trap existed in real life 20 years ago. I hope nowadays no one is using for small networks any more.

  • Knowing your stuff and being good at exams are two different skill sets... Unfortunately sometimes the latter matters more Commented Oct 13, 2019 at 3:56
  • Noone should assume classful addressing in 2019. That is just wrong. Any documentation stating or assuming otherwise is wrong as well. This exam would not be worth the money payed to take it. Commented Oct 22, 2019 at 15:31
  • No, both are networks. The later is also a subnet of the former.
    – Ricky
    Commented Oct 25, 2019 at 16:07

TL;DR is the valid directed broadcast address for the networks

The directed broadcast address for a network has all bits in the address's host part set to 1, so in binary form


works for



\--------network-address--------/\/host-part is not a valid broadcast address for network prefixed to

For people still living in the early 1990s, may imply class A, or /8 in CIDR notation, making it a host address. Network classes were obsoleted in 1993 by RFCs 1518 and 1519 and replaced by Classless Inter-Domain Routing (CIDR).


The address shown are defined in RFCs 1918 and 5771 as reserved addresses for specific use:

a.     is part of (rfc 1918)
b.    is part of (rfc 1918)
c.      is part of (rfc 5771)
d.   is part of (rfc 1918)

In particular, RFC 1918 states that the network is a single number, are 16 contiguous class B (/16) network numbers, and are 256 class C (/24) network numbers. Following that definition, answers (a) and (b) cannot be broadcast addresses, because for (a) should be and for (b) should be Answer (d) is in fact the broadcast address of the network, always taking as basis the document RFC 1918. As of answer (b), RFC 5771 explicitly states that the network is "RESERVED" and its broadcast address is then, so (c) is also wrong. Therefore, the only valid broadcast address present in the list is in answer (d).

Yes, CIDR is the modern way, and Classes are obsolete, but the IETF documentation is the agreed reference for network implementations, and these addresses are defined there.

  • 6
    Even RFC 1918, old as it is, is aware of CIDR and presents 10/8 and friends as "blocks", noting that in pre-CIDR notation, they're just 1x class A network, 16x class B and 256x class C. That doesn't seem to imply that they should only be used as classful networks, not when the document was written, let alone 23 years later.
    – ilkkachu
    Commented Oct 11, 2019 at 21:11

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