Okay, the first subnets you should memorize are the /24 to /30 portion.

/31 and /32 being useless for the

ccna i would say.

So first step, put this in your mind.

- 255.255.255.000 = /24
- 255.255.255.128 = /25
- 255.255.255.192 = /26
- 255.255.255.224 = /27
- 255.255.255.240 = /28 <- MIDDLE of them
- 255.255.255.248 = /29
- 255.255.255.252 = /30
- 255.255.255.254 = /31
- 255.255.255.255 = /32

By having a middle reference, it's easier to figure where you are in the range for faster reverse engineering.

So basicly, if you know that a /28 is .240, you know that a /29 is .248 etc...

Plus the .240 (/28 mask is 16 subnets and 16 hosts ( 14 valid hosts ).

Second good hint

If you get a subnet like

**192.168.0.0 with a mask 255.255.255.192**, without hesitation you must calculate

256 - 192 = 64 hosts per subnets ( subtract 2 for bcast and network)

255.255.255.128 = 256 - 128 =

**128 **- 2

255.255.255.192 = 256 - 192 =

**64 **- 2

255.255.255.224 = 256 - 224 =

**32 **- 2

255.255.255.240 = 256 - 240 =

**16** - 2

and so on...

The networks would be

- 192.168.0.0
- 192.168.0.64
- 192.168.0.128
- 192.168.0.192

The broadcasts addresses would be

- 192.168.0.63
- 192.168.0.127
- 192.168.0.191
- 192.168.0.255

For subnets under /24 it is slightly different because you borrowing bits on the mask.

Let say you have

**192.168.0.0 with a mask of 255.255.192.0**
You also do 256 - 192 =

**64** but this isnt your available hosts, this is your CIDR Step for subnets subdivision.

So every 64 you have a new network or subnet (whatever you call it)

Your networks (subnets) would be:

- 192.168.0.0
- 192.168.64.0
- 192.168.128.0
- 192.168.192.0

Your broadcasts would be:

- 192.168.63.255
- 192.168.127.255
- 192.168.191.255
- 192.168.255.255

The total of valid hosts per subnet would be (2^14) - 2

- 2 being the base 2 ( 0 or 1 )
- 14 being 11111111.11111111.11
**000000.00000000** (255.255.192.0) - -2 being broadcast and network removed.

## Bookmarks