For a Cisco exam, you need to be able to very quickly convert between Decimal, Binary, and Hexadecimal and back again.
Being able to recreate the below tables will greatly help will this
Decimal to Binary and Back Again
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
A decimal number for example 172 is represented in binary as 1 byte which I am sure we all know 1 byte is 8 bits (binary digits). So using the above table if I want to work out what 172 is in binary I simply need to “turn on” the bits from left to right until I make 172.
Example 1: What is 172 in binary?
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
| 1 | 0 | 1 | 0 | 1 | 1 | 0 | 0 |
128 + 32 + 8 + 4 = 172
Example 2: What is 255 in binary?
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
128 + 64 +32 +16 + 8 +4 +2 +1 = 255
Example 3: What is 192 in binary?
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
| 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
128 + 64 = 192
Decimal to Hexadecimal and Back Again
The easiest way to convert between decimal and hexadecimal if to go to IP first, and being able to reproduce the below table during the exam helps, so start learning how to count to 15 in binary!
| Decimal | Binary | Hexidecimal |
| 0 | 0000 | 0 |
| 1 | 0001 | 1 |
| 2 | 0010 | 2 |
| 3 | 0011 | 3 |
| 4 | 0100 | 4 |
| 5 | 0101 | 5 |
| 6 | 0110 | 6 |
| 7 | 0111 | 7 |
| 8 | 1000 | 8 |
| 9 | 1001 | 9 |
| 10 | 1010 | A |
| 11 | 1011 | B |
| 12 | 1100 | C |
| 13 | 1101 | D |
| 14 | 1110 | E |
| 15 | 1111 | F |
As you can see a hexadecimal number represents 4 bits, lets go so converting
Example1: Decimal 176 to Hexadecimal
1) The first thing we need to do is convert 176 to binary using the same method we used before
176 = 10110000
2) A hexadecimal digit can only only be 4 bits so the next step is to break our binary result in two
1011 0000
3) Now we simply use the above table
1011 = B
0000 = 0
4) The answer is
176 = 1011000 = 0xB0
The 0x just lefts everyone know this a hexadecimal number.
Example 2: Decimal 224 to Hexadecimal
1) 224 = 1110000 (128 + 64 +32)
2) 1110 0000
3) 1110 = E 0000 = 0
4) 224 = 11100000 = 0xE0
Example 3: 0xF7 to Decimal
This time we just do things in reverse
1) F = 1111 7 = 0111
2) 11110111 = 247 (128 + 64 +32 +16 + 4 + 2 + 1 or 255 – 8)
3) The answer is 0xF7 = 11110111 = 247
Practice Makes Perfect!
If you would like to have some fun while you practice, check out the game Binary Blitz I wrote while preparing for my Cisco CCNA