## Important numbers to remember as a computer professional

### From SusoSight

When you work with computers a lot, you end up working with a lot of numbers. Often times, to solve or diagnose problems, knowing certain numbers by heart will help you. Here is a list of numbers that I often use in my daily work activities.

**Note:** This is a work in progress of course. Comments are welcome.

## Contents

## Powers of 2

- Powers of 2 - 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, etc.
- The ones that probably come up often are 16, 32, 64, 128, 256, 512, 1024, 4096
^{#1}, 65535 (2^{16}), 16777216 (2^{24}), 2147483648 (2^{31}), 4294967296 (2^{32}), 18446744073709551616^{#2}(2^{64}) - 1 off numbers. When you see a number like 255, you'll know that its 256, but zero based. Or 65535 is 2
^{16}- 1

## Hexadecimal

- Hexadecimal, which is base 16 is usually expressed as 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F. Often you'll see expressions like <A8>, which actually is 169 in decimal.

## ASCII/ANSI characters

Values of various ASCII characters that you may see in the output of programs or used in URLs, etc. I should make a table format for these instead of a list.

### Hex

- 00 - NULL
- 0D - Carriage return
- 20 - Space

### Decimal

- 0 - NULL
- 13 - Carriage return
- 32 - Space

## Time

You'll see a lot of these in DNS TTL records, cron stuff and sleep values.

- 86400 - Number of seconds in a day
- 604800 - Number of seconds in a week
- 3600 - Number of seconds in an hour
- 7200, 10800, 14400 - The first few multiples of 3600.
- 600, 900, 1800 - Number of seconds in 10 minutes, 15 minutes and 30 minutes
- 1970 - The year that Unix epoch time started. Specially, January 1st.

## Turning G into Gi

You often see data values expressed in base 2 instead of base 10. For instance a Kilobyte is actually 1024 bytes, not 1000 bytes. If you ever want to convert some value to the base 2 values, you can just use this formula for some value N to get the result R

- Kibibytes = N/(2
^{10}) - Mebibytes = N/(2
^{20}) - Gibibytes = N/(2
^{30}) - Tebibytes = N/(2
^{40}) - Pebibytes = N/(2
^{50}) - Exbibytes = N/(2
^{60})

To convert the base2 number back to base10 you just reverse the formula and solve for N.

- bytes = Kibibytes*2
^{10} - bytes = Mebibytes*2
^{20} - bytes = Gibibytes*2
^{30} - bytes = Tebibytes*2
^{40} - bytes = Pebibytes*2
^{50} - bytes = Exbibytes*2
^{60}

Or if you already have a base10 quantified number like 10.5 Megabytes and want to express it in Mebibytes, you do this:

Mebibytes = Megabytes * 10^{6}/(2^{20})

The 10^{6} part comes from the fact that 1,000,000 is 10 to the 6th power. A billion or a Gigabyte (base10) is 10^{9} and so on.

This is why that 2 TB drive that you just bought is actually 1.8189 TiB.

## Footnotes

^{12}was 4096 and I mostly remembered it from having an Amiga, which had 12 bit color in HAM mode. Yeah, I'm a geek. I got an applause from the class though and that felt pretty good.