Determining a Subnet Mask??

AustinByrnes

hello,

i am currently halfway through a network + course and have been doing ok so far with all the math, i,e binary, decimal, anding,

i missed a class last week and i am having trouble completing this weeks homework.

i have been asked to determine how many binary bits are required to produce a number of subnet masks for class a,b,c address. (please see attachment)

i have been given a table that has

no. of subnets:2
no. of bits in mask:
subnet mask:
no. of host per subnet:

the only bit of information i have to work out the task is the number of subnets.

to help work out this the trainer has given us the answers to the 1st 2 tasks. i have like 49 to do.7 for class a,b,c, each.

what i am asking help for is given i have the answers to the 1st 2. i need to figure out how i got the answer to the task.so i can then work out the other 47 myself. i can buy a book of answers but i dont want it. it is no use to me in an exam situation if i need to work it out for myself. i want to learn this.

anyhelp is appreicited.

Thanks

NullZer0

Hi there,

Best thing to do is to write those first two answers out as binary and then see what is common between them.

Look at the "subnet bit field" remember that 2^S where S= Subnet bits is the total amount of networks.

So, if I was to ask you how many /24's in a /20 you would say 24-20=4 and 2^4 = 16.

To find how many hosts, its 2^H-2 where H= Number of Hostbits (that is 32 - network bits and then the minus 2 for the broadcast and network addresses which are unusable).

I understand this is confusing at first... it was a huge issue for me years ago when I first tried it the important thing here is to stick at it... the penny will drop after a while ;-)

Heres a publicly available workbook that I used and it really helped.
http://www.scribd.com/doc/2158504/IP-Addressing-Subnetting-Workbook

Check out youtube too, there are some videos there that will help.


HTH,

vibe666

there's 2 things that really helped me get my head around subnetting for the CCNA.

this: http://www.techrepublic.com/article/ip-subnetting-made-easy/6089187

and this: http://www.youtube.com/watch?v=ugANqQprJFI

the 2nd link is a bit cheesy, but once you get your head around the 1st link, the 2nd link makes it relatively easy to do the calculations in your head in just a few seconds.

trebor28

this is a good little game to test you under pressure.
bit like tetris.

http://forums.cisco.com/CertCom/game/binary_game_page.htm