Clarification on subnetting rules with regard to the CCNA

I'm finding some disagreement between different course materials with respect to this following issue.

Does the CCNA exam follow the idea that the number of available subnets with a particular mask is 2 to the n minus 2, i.e. you can't have the first and last subnets of the scheme ?

i.e. if I subnet 192.168.0.0 with 255.255.255.192, do I have 4 available subnets or 2 ?

Because most of the materials I've looked at have said something along the lines of "you used to not be able to use these subnets, but nowadays routers allow them".

I'm finding some of the practice tests I'm using assume that you have to minus out these first and last subnets. It's pretty frustrating when you have given what 'should' be the right answer only to find this particular test sees things differently.

Anyone know what the CCNA exam regards as right ? I feel I really need this clarified before taking the test.

Thanks in advance for your time

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Comments

beh

Someone correct me if I'm wrong, but I was under the impression that (2^n)-2 only had to do with the number of hosts per subnet not the number of subnets.

I would think that there are four available subnets in your example:

192.168.0.0
192.168.0.64
192.168.0.128
192.168.0.192

Each having (2^6)-2=62 hosts.

jscimeca715

beh wrote: »

Someone correct me if I'm wrong, but I was under the impression that (2^n)-2 only had to do with the number of hosts per subnet not the number of subnets.

I would think that there are four available subnets in your example:

192.168.0.0
192.168.0.64
192.168.0.128
192.168.0.192

Each having (2^6)-2=62 hosts.

It's possible that it can be in reference to subnets if the question explicitly states that the no ip subnet zero command is configured. The exam questions should reference whether it is allowed or not, so pay attention the wording of the question and you'll be fine.

MikeO5422

According to Wendell Odom's CCENT/CCNA ICDN1:

When to use which formula for the number of subnets on your CCNA exam.

Use 2^x - 2 if the question includes:

A classful routing protocol or the no ip subnet zero command.

Use 2^x if the question includes:

A classless routing protocol or the ip subnet zero command.

Important* - "If a question simply does not give any clues, assume you can use all subnets" (2^x)

schpenxel

beh wrote: »

Someone correct me if I'm wrong, but I was under the impression that (2^n)-2 only had to do with the number of hosts per subnet not the number of subnets.

I would think that there are four available subnets in your example:

192.168.0.0
192.168.0.64
192.168.0.128
192.168.0.192

Each having (2^6)-2=62 hosts.

there are indeed 4 subnets in this example
192.168.0.0 through 192.168.0.63
192.168.0.64 through 192.168.0.127
192.168.0.128 through 192.168.0.191
192.168.0.192 through 192.168.0.255

first and last of each group of course can not be used. so, you end up with 62 * 4 for total # of usable IP's overall.

cisconooblet

assume the question is for classless on the ccna unless it states otherwise or has a config with no ip subnet zero or a classfull routing protocol.

RoRsChAcH

MikeO5422 wrote: »

According to Wendell Odom's CCENT/CCNA ICDN1:

When to use which formula for the number of subnets on your CCNA exam.

Use 2^x - 2 if the question includes:

A classful routing protocol or the no ip subnet zero command.

Use 2^x if the question includes:

A classless routing protocol or the ip subnet zero command.

Important* - "If a question simply does not give any clues, assume you can use all subnets" (2^x)

This is sound advice for the test!

Maritz

Thanks very much for this.

lon21

MikeO5422 wrote: »

According to Wendell Odom's CCENT/CCNA ICDN1:

When to use which formula for the number of subnets on your CCNA exam.

Use 2^x - 2 if the question includes:

A classful routing protocol or the no ip subnet zero command.

Use 2^x if the question includes:

A classless routing protocol or the ip subnet zero command.

Important* - "If a question simply does not give any clues, assume you can use all subnets" (2^x)

So in rip v1 you can't use the first and last subnet?