[nycbug-talk] IPv6 and CIDR confusion

alex at pilosoft.com alex at pilosoft.com
Fri Mar 23 21:12:42 EDT 2007

On Fri, 23 Mar 2007, Isaac Levy wrote:

> The IPv4 CIDR number comes from the number of 1's in the subnet mask
> when converted to binary, right?
Nope. That's the number of bits in the *address* part of the prefix, just 
like v6.
> So it seems the IPv6 notation is not the same thing at all- it's not the
> subnet mask, but the prefix length of the address.
Same as v4
> That was confusing, now it's clear.
> >
> > To have 254 usable IP addresses, you can have a /120.
> Alex- do you do the hexadecimal counting in your head dude?!?!?
Very simple math:
You need 8 bits to address 256 things.
IPv4 is 32 bits address space. 32 - 8 = 24, thus you need /24 prefix.
IPv6 is 128 bits address space. 128 - 8 = 120.

> > Now, the interesting thing is, most v6 capable routers only will route
> > on the top 64 bits of the prefix.
> - whaddya' mean?  That sounds interesting?
Basically, this means that you can't have netmasks longer than /64. I.E. 
local subnet will be always /64 or longer.

Example, if you are allocated /48 worth of address space, the most 
*networks* you can break it down is 2^(64-48) = 65536 networks.

> Rocket- .ike
> (p.s.: for a guy who seems so hellbent against IPv6, you sure know a  
> lot of details...)
I'm not against ipv6. I'm just explaining that its not here, yet.


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