SusoSight 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.

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 (2¹⁶), 16777216 (2²⁴), 2147483648 (2³¹), 4294967296 (2³²), 18446744073709551616^#2 (2⁶⁴)
• 1 off numbers. When you see a number like 255, you'll know that its 256, but zero based. Or 65535 is 2¹⁶ - 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/(2¹⁰)
• Mebibytes = N/(2²⁰)
• Gibibytes = N/(2³⁰)
• Tebibytes = N/(2⁴⁰)
• Pebibytes = N/(2⁵⁰)
• Exbibytes = N/(2⁶⁰)

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

• bytes = Kibibytes*2¹⁰
• bytes = Mebibytes*2²⁰
• bytes = Gibibytes*2³⁰
• bytes = Tebibytes*2⁴⁰
• bytes = Pebibytes*2⁵⁰
• bytes = Exbibytes*2⁶⁰

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 * 10⁶/(2²⁰)

The 10⁶ part comes from the fact that 1,000,000 is 10 to the 6th power. A billion or a Gigabyte (base10) is 10⁹ and so on.

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

Footnotes

[#1] - One day in a college level calculus I blew everyone away when I knew that log2 of 4096 was 12 within a second or two without using a calculator. I simply knew that 2¹² 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.

[#2] - 18446744073709551616? What you can't be serious. No, I don't expect you to remember that whole number, but usually you'll just recognize it by it starting with 1844 and then a long string of digits and you'll know that its probably something to do with 64-bit boundaries.