CCNA: Ip Subnet-Zero


1) This is related to the CCNA Exam.
2) This discussion is taken from one of the prep test.
The given Question:
Please study the exhibit (attachment) carefully. All of the Boba routers in the Boba network are configured with the ios command " ip subnet-zero ". Because of this, which network addressess should be used for LINK Boba and NETWORK Boba? (Choose two)
The given Answers:
Network Boba:
LINK Boba:
3) My Questions:
First: What is ip subnet-zero? (Please explain it with your own wordings firstly, and then you can provide the link if any)
Second: What is the different between LINK and NETWORK?
Third: Why the given answer of the LINK Boba is (would you explain it please)
4) Thank you


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Ahmed Ezzat AbuRayaNetwork Developer EngineerCommented:
1)  zero-subnet:
With old subnetting rules, the all 0's subnet was reserved for network (and the all 1's subnet was reserved for broadcast).
Then later on, they found out that the zero-subnet is not really used, so it could be considered a useable network.

An example is:  (SNM: <-- this is an IP address using a zero subnet.  See the 0 in the third octet? Even if it looks weird, this is a legal address.

So if you have an IP address of with SNM:, using zero-subnet, you can actually get 255 valid networks out of it (10.1.{0-254}.X) where X represents hosts 1-254..   So it gives room for networks (0-254) = 255 total networks.

2) Difference between LINK & Network

I think it is because the LINK here is a point-to-point network.. (e.g. subnet used is /30 which gives you 4 hosts.. -, you have ( as non valid hosts. so this leaves you with two valid hosts. hence it is a link to the two hosts.
The other is called a network, because it is a network (e.g. more than 2 valid hosts)

3) It is a practical use of zero-subnet.  you have the following networks (and links)  (4 hosts)  (are you sure this is as the pic?
or I just can't see.. ?)  (4 hosts) (4 hosts) (32 hosts) (32 hosts) (32 hosts)
So to make use of the ip subnet-zero you use:  (4 hosts)

4) Hope I could help :)

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Don JohnstonInstructorCommented:

What Zo3FoL is saying is mostly correct. However some of the examples are not. is not using subnet zero. An example of subnet zero use would be

In order to qualify as using subnet zero, all of the bits in the subnet field have to be zeros.

Another example of using subnet zero would be

And I agree that "link" is used to reference a point-to-point connection.

As for the /30 address allowing 4 hosts, this is not correct. A 30 bit mask will allow 2 hosts. On every IP network, there are two addresses that are not used for hosts. All zeros (network address) and all ones (broadcast). Zo3FoL does mention this but in his examples, there are (4 hosts) after it.
Ahmed Ezzat AbuRayaNetwork Developer EngineerCommented:
tjie, what I meant is that /30 address gives a block size of 4 .. (two of them are not valid, while only 2 are :) As I explained in point #2)

Though thanks for clearing this out.

Best regards :)
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