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When I first heard about IPv6, I just assumed that it doubled the number of addressing bits in IPv4, from 32 bits to 64 bits. This would have been enough for at least 2.6 billion addresses for each person on the planet. I just found out that the address size was actually quadrupled to 128 bits, which is an astonishing 4.8e+28 addresses per person.

Why was 128 bits chosen?

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closed as not constructive by Dennis, Diogo, haimg, ChrisF, studiohack Jan 18 '12 at 17:21

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They originally thought 32bit would last forever, I guess they overcompensated for this misjudgment and went 64. –  Moab Jan 18 '12 at 15:41
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Bill Gates once quotes "Nobody will need more than 4mb of RAM", and alas when IPv4 was made they thought they would never get close to the limit.. lets learn from history and today they say .. Lets see who can break these barriers!? We Challenge you! yea 2.6billion IP's for each person on the planet times 1 billiion people OMFG! By then Aliens would have introduced Neural networking.. should be enough till then.. –  ppumkin Jan 18 '12 at 15:45
    
@Moab, 64 would have been reasonable. Instead they chose 128??? –  Mark Ransom Jan 18 '12 at 15:45
    
@ppumkin, obviously I wasn't clear in my question. 2.6 billion should indeed be enough, but they decided to go much much higher than that. –  Mark Ransom Jan 18 '12 at 15:46
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It's a shame that this was closed for supposedly being impossible to answer factually. The history of the evolution of IPv6, and the decision process that went into the address size choice, is in fact fairly well documented. –  JdeBP Jan 18 '12 at 17:34

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They were looking at 64 bits of address space for a while, and how many addresses per square foot/meter that meant. Then they looked at population density, in very high density places. They saw that in a dense highrise in say NYC or Shanghai, you actually come close to that limit. What would be the point of the rollout if you may need to increase size again in the easily foreseeable future.

So they went to 128 bits, so that there was no chance at all you'd exceed that. Though... (obligatory XKCD joke)

It also helps with routing. Think of all the madness with address classes, CIDR, and all that. By having some extra bits, you can make the network/machine split part of the address cleaner.

I'm reminded of Animal House .. "Why Pinto?" "WHY NOT?!". 128 bits puts 8 more bytes in the packet. It's not a huge increase in size. When ipv4 was being invented around 150bps or 300bps was pretty common. Now I have 12MBps to my home, and the US is actually pretty slow when it comes to broadband (compare to say, South Korea). An extra 8 bytes per packet to futureproof it isn't all that bad.

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I'm confused about how physical density would enter into the deliberations - there's nothing about IP addresses that are limited geographically. P.S. Thanks for the XKCD link. –  Mark Ransom Jan 18 '12 at 21:34
    
@MarkRansom 1) there are correlations between geography and IP address, routers and such are at a single location. GeoIP does exist and works. 2) it's about constraints. I test one area, find the number of IP addresses per meter lacking, so i know if i scale up, I get exhaustion again. remember, before we thought we'd need so few addresses, normal companies (HP, Apple) got class A address spaces. We didn't guess expansion well, so saying "only New York will ever be that dense" didn't make them confident. –  Rich Homolka Jan 19 '12 at 20:40

Because you never know what the future brings. Imagine every device in your home having its own IPv6 address. That way, a lot more addresses would be needed than you would think right now.

So basically, it's future thinking.

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As long as I don't have 2.6 billion devices in my home, 64 bits would have been plenty. –  Mark Ransom Jan 18 '12 at 15:51
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@MarkRansom: In 50 years everything in your house could be made out of computronium. –  Zan Lynx May 14 '12 at 21:18

IPv6 address space is typically handed out in multiples of /64 blocks to individuals. This makes it easier to configure subnets and autoconfiguration based on MAC address.

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