xls for IP Subnetting study and memorization
We all do it, and it is best to memorize the class 4 IP subnet’s.
Link below:
Also this chart is very good to memorize and write out. If you know the block sizes and the /’s, you have 90% of what is needed.
| Subnet | Block | A | B | C |
| 0 | 256 | 8 | 16 | 24 |
| 128 | 128 | 9 | 17 | 25 |
| 192 | 64 | 10 | 18 | 26 |
| 224 | 32 | 11 | 19 | 27 |
| 240 | 16 | 12 | 20 | 28 |
| 248 | 8 | 13 | 21 | 29 |
| 252 | 4 | 14 | 22 | 30 |
| 254 | 2 | 15 | 23 | 31 |
| 255 | 1 | 16 | 24 | 32 |
Also this “Cheat sheet” has been around forever. Very good reference http://www.aelius.com/njh/subnet_sheet.html
Once you get the process down the “IP Subnet Calculator” makes a great reference http://www.subnet-calculator.com/
Nice chart! One quick note, however: Maybe there’s not a real good reason from a memorization point of view, but just for completeness I’d have included the /31 subnet. Just remember to enable directed broadcast if you use /31 for point-to-point links. This is now the Cisco recommended configuration for true point-to-point.
Like I said, though, great chart.
Thanks! Good idea, will add that. Was going to fix this up a little anyhow.
For memorizing realized you only really need to know a lot less than on that sheet. But the sheet is a great reference. Have a large on printed in my office.
My new method for memorizing IP Subnetting.
Remember the bit values in the “interesting octet” used to subnet (128,64,32,16,8,4,2,1).
You add these left to right and get the subnets. 0 is the only odd one, just remember always start with 0.
Subnet Bits (# of subnets) always starts at 1 and is powers of 2.
Block Size is always Subnet – 256.
Host Bits (# of hosts) are all the 0’s after the last Subnet Bit to power of 2. +8 for Class B, and +16 for Class A.
Class C Example, but only # of Hosts changes for A (+16) & B (+8).
(Subnet), (# Subnets), (Block Size), (# of Hosts)
0, 1, 256, 265
128, 2, 128, 128
192, 4, 64, 64
224, 8, 32, 32
240, 16, 16, 16
248, 32, 8, 8
252, 64, 4, 4
254, 128, 2, 2
Just have to remember that 0 always starts at /8, /16, or /24.
/8 = Class A
/16 = Class B
/24 = Class C
And we just at +1 to each we go down the line.
As you can tell I am studying for CCNA. Think if a person remembers this and writes it out on the laminated posters they give for scratch paper should be good.
Craig