Cisco's favorite subnet mask?

I'm studying my subnetting right now and I was wondering if it's true that Cisco's favorite subnetting questions are based off the subnet mask of 240? In my Todd Lammle book, he makes a note that Cisco noticed people have a hard time counting in increments of 16 so they tend to use subnetting questions with that subnet mask. Say they ask if a ip of 198.168.1.95/28 is a valid usable host address.

Also, are VLSMs in ICND1 or is it ICND2 only?

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Comments

JavonR

Meh... I think thats speculation, all questions are random.

There is no VLSM on the ICND1, thats an ICND2 topic.

rbutturini

The old exam seemed to have a lot more of the tricky subnetting questions. The newer exam seemed to be more "real world" Agreed though, it's pretty much a crapshoot as far as what questions you get.

Talic

rbutturini wrote:

The old exam seemed to have a lot more of the tricky subnetting questions. The newer exam seemed to be more "real world" Agreed though, it's pretty much a crapshoot as far as what questions you get.

That's good to hear because 16 is a real pain to count up to. If you wanted to find out if 198.168.10.174/28 was a valid host address, not a broadcast or network address, would take forever.

So far I think I have it down except I got to work on finding out what are valid hosts, not network or broadcast addresses.

JavonR

It's really not that bad with some practice, you just need to remember a few key #'s and either count up or count down. For example with .174, I know that 160 is a multiple of 16 (16X10), therefore the next subnet would be 176 (160+16) making .175 the broadcast address of the .160 network and .174 a valid host.

tiersten

Learn to subnet properly. You don't want to assume that you'll only get questions of a specific type.

yuriz43

Increments of 16 are smack in the middle. 4 prefix bits in the interesting octet. This does make it the hardest to calculate.

What I did on my ccent test was write down the following so that I could easily find the ranges IP addresses in a given subnet mask.

16
32
48
64
...
...
...

jbriggy

Talic wrote:

Also, are VLSMs in ICND1 or is it ICND2 only?

Actually both, just to different extents in each level. With ICND1, you'll need to know "Which subnet mask will give you X hosts and Y networks" With ICND2, you'll need to know that, as well as WHERE to assign them in a network topology, and how to summarize routes efficiently.