subnet prefix length > 64 breaks IPv6?

Karl Auer kauer at
Wed Jan 4 02:41:41 UTC 2012

On Tue, 2012-01-03 at 15:45 -0800, Owen DeLong wrote:
> Technically, link local is fe80::/10, though many implementations erroneously
> treat it as fe80::/64. In most cases, since the 54 bits between fe80 and the
> IID are almost always 0, this error has no impact.

Yes, well, I'm a bit confused about that. Maybe I haven't read the trail
of overlapping, obsoleting and conflicting RFCs carefully enough.

RFC 4862 (section 5.3) says that the interface ID can run all the way up
to the end of the link-local prefix. Since this is defined as a /10, an
interface ID can be up to 118 bits long. In RFC 4862 the prefix length
is not actually given; instead it says "the well-known link-local prefix
FE80::0 [RFC4291] (of appropriate length)".

RFC 4862 also says that the whole thing must be consistent with RFC
4291. RFC 4291 (section 2.5.6), defines the first ten bits as
1111111010, then the next 54 bits as zero - BUT does not specify a
prefix length. Those implementations that use /64 can thus be forgiven,
I think.

So - are those 54 bits reserved and zero, or can an interface ID be
anything up to 118 bits long? I'd be interested in a definitive answer,
if there is one.

Regards, K.

Karl Auer (kauer at

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