Binary Conversion/Subnets/Subnet Mask Chart
Bits
Decimal Value
Decimal Mask
Binary Mask
# of Subnets
Subnet Breakout
High/ Low Order
1
128
NA
NA
0
NA
High Order Bit
1
64
192
1
32
224
1
16
240
1
8
248
1
4
252
1
2
254
1
1
255
11000000
11100000
11110000
11111000
11111100
11111110
11111111
2
.64-
6
.32-
14
.16-
30
.8-
62
.4-
126
.2-
254
.1Low Order Bit
* How to reproduce the chart above
 Decimal value starts at 1 for low order bit and doubles until 128 (the high order bit)
 Decimal mask is a sum of the decimal values of the bits you are using (starting from the high order bit)
 # of subnets is calculated by assigning a value of 0 to the mask for the high order bit, and then doubling and adding
two as you go to the right
 Subnet breakout is the range of IP addresses you can assign: increment by the number you start with, start with the
number that is the decimal value of the low order bit of your subnet mask.
Tip: Determine the number of bits you need for the network and the number of bits you need for the hosts and then use
this equation to find the decimal equivalent:
2bits -2
Scenario 1: You want X number of subnets, what is the subnet mask?
 Look at the chart above –or Convert X to binary, determine how many low-order bits you need to make the number (can’t have all 1’s; if number is
all 1’s add another bit), that many bits is the number of high-order bits that make up your subnet mask, convert the
high-order bits to decimal value –orScenario 2: Determine what the range of valid address is (subnet breakout) for X subnet mask
 Look at the chart above –or Convert X to binary and determine the decimal value of the lowest high-order bit, start the range of addresses at that
value, and increment the range by that value
Scenario 3: Convert binary to decimal and decimal to binary
 Start at the low-order bit, it’s value is 1, the value of every bit to the left is double the one to the right of it, add all the
values where there is a one in the binary number, disregard the zero’s. –or Use the scientific view on the Windows calculator to convert either way
Scenario 4: You want X number of hosts and X number of subnets
 Look at the chart above for the subnets, then use the equation 2bits –2 to determine the number of hosts (use the
number of bits left for hosts) –or Convert X (for the subnets) to binary, determine how many low-order bits you need to make the number (can’t have
all 1’s; if number is all 1’s add another bit), that many bits is the number of high-order bits that make up your subnet
mask, convert the high-order bits to decimal value- that is the number of subnets; take the number of bits left and
convert that to decimal and subtract 1 – that is the number of hosts
Scenario 5: Determine the # of subnets for X subnet mask
 Look at the chart above –or Convert X to binary and use the equation 2bits –2 to determine the number of subnets
Class A Addresses
Class B Addresses
Class C Addresses
Class D Addresses
Class E Addresses
1-127.x.x.x
128-191.x.x.x
192-223.x.x.x
224-239.x.x.x
240-254.x.x.x