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Home > Archive > CCNA > April 2003 > subnetting example
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subnetting example
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| djmaplethorpe 2003-04-22, 2:35 pm |
| This is an example out of the Sybex book and want to make sure I'm seeing this right.
Network Address 172.16.0.0
Subnet Address 255.255.240.0
= subnets 2^4-2=14
= hosts 2^12-2=4094
Valid subnets 256-240=16, 16+16=32 and so on.
in Binary this would translate to 1111 for the network portion and 0000 00000000 for the host portion.
The list of subnets shown are 16.0, 32.0, 48.0, and so on.
The first host = 16.1, 32.1, 48.1
Last host = 31.254, 47.254, 63.254
Okay my qustion is when I get done with the 16's and move to the 17's I am suppose to borrow from the next four in the same octet?
This is the way I am seeing it and want to make sure I have it. I only used the binary as an easier visual to the question. | |
| anchor40 2003-04-22, 2:48 pm |
| When you supernet, think of the resulting address range as a pool of binary numbers to utilize however you see fit. To use your example:
0000 00000000 is the network
1111 11111111 is the broadcast
and all in-between are valid host addresses.
so 17.0/20 is a valid host id, as is 17.255/20, but not 31.255/20 which is the broadcast address for this network.
clear as mud? 
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| izuki 2003-04-22, 11:00 pm |
| thanks, anchor40.
your explanation is easy and clear to understand. | |
| anchor40 2003-04-23, 8:24 am |
| Thanks! Glad I could help.
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| darronb 2003-04-28, 8:40 am |
| !!!quote:
so 17.0/20 is a valid host id, as is 17.255/20
I am a little confused, isn't 17.255/20 an broadcast address
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| djmaplethorpe 2003-04-28, 9:54 am |
| quote:
Originally posted by darronb
!!!
I am a little confused, isn't 17.255/20 an broadcast address
With the mask provided it would be considered a host address as the range of addresses goes from 16 to 32 as in 16.0 to 32.0 so 17.255 with a mask of /20 would be host. nnnnnnnn.nnnnnnnn.nnnnhhhh.hhhhhhhh. n's being network and h's being host portions. | |
| anchor40 2003-04-28, 10:13 am |
| The network and broadcast addresses are the most tricky with CIDR.
Best way to clarify is by drawing your bit-boundaries from the subnet mask provided (like I did above) and write out the valid host range.
A hint, zero can never be a host if the mask is /24 or higher (/25, /26, etc), but can be if the mask is /23 or less.
Subnet calculators can help you in real life, but just like in 3rd grade math class, you can't use a calculator on the test! The only way to overcome is to practice until you're comfortable!
I'm going to start a poll on this subject, and may offer "subnetting school" questions on this forum for those that would like some extra practice, if the poll warrants. Look for it, and vote, if you're so inclined.
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