1. CCNA4.com
Got this off another site but it's useful here also:
To remember the subnetting tables all you have to do is start with "4" and double it until you
get to "16384" Write them downward on a sheet of paper and when you are done just subtract
2 from each number.
ie:
4=2
8=6
16 = 14
32 = 30
64 = 62
once you have done that all you need to do is reverse the order of all the numbers going back
up the sheet:
subnets hosts
2 62
6 30
14 14
30 6
62 2
See how the numbers flip flop between each column? My example is for class C but it works
for class B just the same.
Once you have the subnet/host numbers written out, just remember the following numbers
.192, .224, .240, .248, .252
class C:
sub hosts
.192 /26 2 62
.224 /27 6 30
.240 /28 14 14
.248 /29 30 6
.252 /30 62 2
The numbers with a slash (ie /26) are just short hand ways of writing out subnets. They can be
really confusing if you are trying to learn subnetting for the first time. Just rember that ip
addresses are made up of 32 bit addresses, or /32. These 32 bit addresses are broken down
into class A,B, and C. class B are from /18 to /30 and class C go from /26 to /30. The reason
the numbers don't go up to /32 are because it goes against the rules of subnetting (according to
Cisco), I don't have any other reason why.
You need to memorize this stuff!!! When I went to work I jotted notes all over my desk and
tool boxes just so I would see it all the time.
Here is the class B example:
(1). start with 4 and double it till 16384: 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096,
8192, 16384.
(2). subtract 2 from each number: 2, 6, 14, 30, 62, 126, 254, 510, 1022, 2046, 4094, 8190,
16382.
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2. CCNA4.com
(3). write them downward on a sheet of paper and then write them back up in reverse order:
2 16384
6 8190
14 4094
30 2046
62 1022
126 510
254 254
510 126
1022 62
2046 30
4094 14
8190 6
16382 2
(4) Finally you just have to add the net number to your list... Rember these numbers: .192.0
(/1) .224.0 (/19) .240.0 (/20) .248.0 (/21) .252.0 (/22) .254.0(/23) .255.0 (/24) .255.128 (/25)
.255.192 (/26) .255.224 (/27) .255.240 (/28) .255.248 (/29) .255.252. (/30)
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