Every YouTube video has a unique ID.

It’s up in the URL: a string of eleven characters

that uniquely identifies which video you want.

Now, YouTube has millions and millions of

videos.

The last stats that they released said they

have

400 hours of video being uploaded every minute.

So: are they ever going to run out of those

IDs?

Well, to find out, let’s talk about counting

systems.

People count in Base 10. 0 to 9.

That’ll be, hopefully, familiar to you.

Computers count in base 2, in binary,

but that’s difficult for humans to read,

it gets too long to write really, really quickly,

so often computers will display it in base

16, hexadecimal.

You have 0 to 9, and then A to F,

and then you start adding to the next column.

Humans can’t understand that easily,

but it’s efficient if we have to type it in

somewhere,

and 16 – 2 to the power of 4 – is also easy

for computers to deal with.

So how about Base 64?

That’d be a ridiculous counting system, right?

Except.

64 is another one of those easy numbers for

computers,

it is 2 to the power of 6.

And humans can get to 64 very easily:

0 to 9, then capital letters A to Z,

then small letters a to z, and two other characters.

Most Base 64 uses slash and plus,

but they don’t work so well in URLs,

so YouTube uses hyphen and underscore.

That YouTube URL, that unique ID,

is really just a random number in base 64.

They could have have picked base 10 or base

16,

but they didn’t: they went with 64,

because it will let you cram a huge number

into a small space

and still make it vaguely human readable.

Author and programmer Sam Hughes, by the way,

pushed this to the limit, and invented Base

65,536,

which includes basically every character from

every language.

It is ridiculous and unnecessary,

but when has that ever stopped programmers?

So why didn’t YouTube just start counting

at 1 and work up?

Well, first, they would have to synchronise

their counting

between all the servers handling the video

uploads,

or they’d have to assign each server a block

of numbers.

Either way, there’s a lot of tracking to do,

a lot of making sure that it’s never duplicated.

Instead, they just generate a random number

for each video,

see if it’s already taken, and if not, use

it.

And secondly, it is a really, really bad idea

to just count 1, 2, 3 and so on in URLs.

Incremental counters, as they’re called, can

be a big security flaw:

if you see video 283 up there, then you might

wonder:

what’s video 284? Or video 282?

It’s easy to enumerate, as it’s called,

to run through the entire list.

YouTube Unlisted videos, the ones that don’t

appear publicly

but that you can send the link to people,

those wouldn’t work.

And by the way? Lots of badly designed sites

do use incremental counters.

And it is a terrible idea.

It might tell your competitors exactly how

many customers you have,

‘cos they can just count them.

It might let people download all your records

easily,

‘cos they can just run through them.

And in one site that someone in Florida emailed

me about this week,

it lets you look at other people’s personal

details.

Don’t use incremental counters if you’re building

a web site. Use a random number.

Which brings me to the question:

just how big are the numbers that YouTube

uses?

Well, let’s work it out.

One character of base 64 lets you have 64

ID numbers.

Two characters? That’s 64 by 64, or 4,096.

Three characters? 64 times 64 times 64 — or

64 to the power of 3.

That is already more than a quarter of a million.

And if we go to four? Well, now we’re above

16 million.

If you use Base 64, then you can assign an

ID number

to everyone who lives in London down there

twice over,

and you’ll only need four characters.

This gets big fast. We can keep on doing this,

and by seven characters we’re already at four

quadrillion.

Now, I assume that YouTube checks through

a dictionary,

and doesn’t allow any actual words to appear

up there —

particularly anything rude.

But that is going to be a tiny minority of

the URLs,

so for our purposes, we can pretty much just

ignore that.

At YouTube’s 11 characters, we are at 73 quintillion

786 quadrillion

976 trillion 294 billion 838 million

206 thousand and 464 videos.

That’s enough for every single human on planet

Earth

to upload a video every minute for around

18,000 years.

YouTube planned ahead.

Can they run out of URLs? Technically, yes.

Practically? No. And if they did?

They could just add one more character.

[Translating these subtitles? Add your name here!]

Ha! One take! One take! Yes!

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