A couple of subnet questions...rounding??

sleemiesleemie Posts: 109Member
not asking anyone to break the dislosure thingy, but for the purposes of the test how much rounding can I get away with subnetting? I realize when calculating the number of subnet IDs you have to be precise, but when calculating the number of hosts how much can I get away with? for example, I know a class b network starts out with a max of 65,534 hosts, if i'm gonna work a problem in my head can I just say it starts with 65,000, if I take one bit it's 32,500, two bits it's 16,000, three bits it's 8,000 and so on. or if I'm doing the 2s thing I know 2 to the 10th gives me 1,024, can i just go with 1,000 and backing up to the 9th would be 500, the 8th 250, and so on?

I know in the real world you would need to be precise, but you would have the benefit of the extra few minutes get that info.

Also, what chart should I memorize and jot down at the begining of the test?

Just thought of another question while doing some practice questions..how do you quickly figure out the subnet IDs when you have a higher end mask like 248- 254 and they're asking you about host addresses in the upper ranges. for example, if the address is making the mask 252 and the subnet IDs in increments of 4 and they throw out as possible host addresses and, etc. it would take a long time just starting at 4 and having to count up to that range to figure the subnet IDs.


  • Joe A.Joe A. Posts: 8Member ■□□□□□□□□□
    I STRONGLY suggest that you don't round down.If you know that subnets have to be precise,why then would you round down on hosts?You know that 2^10=1024,then just cut it in half for 2^9=512,2^8=256...and so on,PRECISION.These numbers have to be right on.If you have not reached VLSM yet,you'll see that precision is of the utmost priority.
  • Joe A.Joe A. Posts: 8Member ■□□□□□□□□□
    To add to the second part of your question,it would take awhile counting by fours from zero,but,there are constants that all the block sizes{2,4,8,16...} fit into.If I'm faced with that kind of question instead of starting at the bottom 0,4,8... Ijump to one of the constants that is closest to that subnet. Example: block size of 4,so instead of starting at {0,4,8,12...} Iwould start at constant 224,because 224 is a constant in ALL block sizes.These are the constants that i use


    If you memorize these you'll never have to waste time counting from the bottom up just jumpto the closest number and start from there.
  • pizzafartpizzafart Posts: 97Member ■■□□□□□□□□
    I agree with Joe.

    Another thing I find helpful is using a factor of the denomination. Let's say your address is and your subnet mask is

    Here your denomination is eight. Rather than starting from 0 and counting up, you can look for a multiple of 8 that's easy to do in your head and that doesn't pass 102. An easy one is 8 by 10 = 80. At this point you could count up by eights: 88, 96, 104 ... whoops, passed it, and now you know that the subnet address is

    It's definitely important to have subnetting and address allocation down. I'd practice enough so that most any question comes pretty quick. Memorizing a few lists in the process is a good idea. I've passed 3 cisco exams at this point and every one of them has had a fair number of subnetting questions.
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