subnet prefix length > 64 breaks IPv6?

Sven Olaf Kamphuis sven at
Sat Dec 24 15:30:10 UTC 2011

it only breaks the auto configure crap which you don't want to use anyway.

(unless you want to have any computer on your network be able to tell any 
other computer "oh hai i'm a router, please route all your packets through 
me so i can intercept them" and/or flood its route table ;)

we use all kinds of things from /126'es to /112 (but hardly any /64 crap)

works perfectly fine.

as long as its nibble aligned (for other reasons ;)


Sven Olaf Kamphuis,
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On Sat, 24 Dec 2011, Glen Kent wrote:

> Hi,
> I am trying to understand why standards say that "using a subnet
> prefix length other than a /64 will break many features of IPv6,
> including Neighbor Discovery (ND), Secure Neighbor Discovery (SEND)
> [RFC3971], .. " [reference RFC 5375]
> Or "A number of other features currently in development, or being
> proposed, also rely on /64 subnet prefixes."
> Is it because the 128 bits are divided into two 64 bit halves, where
> the latter identifies an Interface ID which is uniquely derived from
> the 48bit MAC address.
> I am not sure if this is the reason as this only applies to the link
> local IP address. One could still assign a global IPv6 address. So,
> why does basic IPv6 (ND process, etc) break if i use a netmask of say
> /120?
> I know that several operators use /120 as a /64 can be quite risky in
> terms of ND attacks. So, how does that work? I tried googling but
> couldnt find any references that explain how IPv6 breaks with using a
> netmask other than 64.
> Glen

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