Using /126 for IPv6 router links

Richard A Steenbergen ras at
Mon Jan 25 10:45:01 UTC 2010

On Mon, Jan 25, 2010 at 09:12:49AM +0000, Andy Davidson wrote:
> There are 4,294,967,296 /64s in my own /32 allocation.  If we only ever
> use 2000::/3 on the internet, I make that 2,305,843,009,213,693,952
> /64s.  This is enough to fill over seven Lake Eries.  The total amount
> of ipv6 address space is exponentially larger still - I have just looked
> at 2000::/3 in these maths.

Don't get carried away with all of that "IPv6 is huge" math, it quickly
deteriorates when you start digging into it. Auto-configuration reduces
it from 340282366920938463463374607431768211456 to 18446744073709551616
(that's 0.000000000000000005% of the original 128 bit space). Now as an
end user you might get anything ranging from a /56 to a /64, that's only
between 1 - 256 IPs, barely a significant increase at all if you were to
actually use a /64 for each routed IP rather than as each routed subnet.
As a small network you might get a /48, so that even if you gave out
/64s to everyone it would be only 16 bits of space for you (the
equivalent of getting a class B back in IPv4 land), something like a
8-16 bit improvement over what a similar sized network would have gotten
in IPv4.  As a bigger ISP you might get a /32, but it's the same thing,
only 16 bits of space when you have to give out /48s. All we've really
done is buy ourselves an 8 to 16 bit improvement at every level of
allocation space (and a lot of prefix bloat for when we start using more
than 2000::/3), which is a FAR cry from the 2^128 "omg big number, we
can give every molecule an IPv6 address" math of the popular
imagination. :)

Richard A Steenbergen <ras at>
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)

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