When working with IP addresses it is very handy to have a cheat sheet available in order to quickly calculate netmasks and for converting to/from CIDR notation.  So here is a quick chart that is printable for wall hanging.

Here is some explanation on how the chart is presented:

The first row in the chart is the decimal representation of each placeholder in an 8 bit (binary) number.  This is pretty self explanatory, nothing so far that you would learn outside of math class.

The second row is the netmask equivalent for each decimal placeholder value. 

Let’s start with a decimal representation of a subnet mask:

x.x.x.x – Where x equals a number between 0 and 255 – well, actually it can’t be any number between 0 and 255 when we are talking about netmasks.  To clarify, in a netmask, the x can only be one of the following numbers: 0, 128, 192, 224, 240, 248, 252, 254, or 255.  Each x represents one octect and we know (version 4) IP addresses and subnet masks each have a total of four octects.

The netmask value is the inverse value of the decimal number.  To come up with this value we take the number 256 (which is how many numbers we can get from a binary 8 bit number) and we subtract the decimal value from it and that gives us the netmask equivalent. 

The same conversion in binary would look like the following:

The inverse value of 00001111 (which is a decimal 16)  would be 11110000 (a simple flip, ones become zeros and zeros become ones), which is a decimal 240.

The remainding lines represent the CIDR notation of a given netmask value.  The CIDR value represents how many binary ones are represented in a given netmask.  Let’s go back to the decimal representation of a netmask:

255.x.x.x – The class A boundary would be between the first and second octect.  There are inherantly 8 binary ones in this 32 bit binary number – before any additional subnetting is applied.
255.255.x.x – The class B boundary would be between the second and third octect.  There are inherantly 16 binary ones in this 32 bit binary number – before any additional subnetting is applied.
255.255.255.x – The class C boundary would be betwen the third and fourth octect.  There are inherantly 24 binary ones in this 32 bit binary number – before any additional subnetting is applied.