[Dnsmasq-discuss] Cannot assign IPv6 address for /96 subnet

Sheng Yang sheng at yasker.org
Wed Feb 13 04:36:55 GMT 2013


On Tue, Feb 12, 2013 at 6:48 PM, Brian Haley <brian.haley at hp.com> wrote:
> On 02/12/2013 05:43 PM, Sheng Yang wrote:
>> I am also a little dubious here, but if people want to get some
>> smaller subnet, do they able to get it anyway on IPv6?
>
> You typically use a /64 on a link, but you can use longer.  The problem is that
> address auto-configuration will only work if the prefix length is 64 since that
> plus the IID length of 64 == 128.  In your case, 96 + 64 is simply too large.
>
> A Linux system receiving a !64 prefix length will log this error in
> /var/log/kern.log:
>
>         IPv6 addrconf: prefix with wrong length 96
>
> That would confirm my assumption.

The auto generated link local would be always /64, but seems irrelevant here.

We didn't use autoconf here, because we need to know the IP we
assigned from dhcp server. So lost autoconf ability seems fine to me.

I just don't know if it's legal even when we use stateful dhcpv6 to assign IPs.

--Sheng

>
> -Brian
>
>> On Tue, Feb 12, 2013 at 1:06 PM, Brian Haley <brian.haley at hp.com> wrote:
>>> On 02/08/2013 09:59 PM, Sheng Yang wrote:
>>>> Hi Simon,
>>>>
>>>> I found I can't assign IPv6 address for /96 subnet.
>>>>
>>>> I specified DHCP range use 96 bits prefixes:
>>>>
>>>> dhcp-range=fc00:3:1602::7473,96,static
>>>
>>> This is in violation of RFC 4862 Section 5.5.3 Item d, right?
>>>
>>>     "If the sum of the prefix length and interface identifier length
>>>      does not equal 128 bits, the Prefix Information option MUST be
>>>      ignored."
>>>
>>> Basically, anything besides a /64 on an Ethernet isn't going to work.
>>>
>>> Unless I mis-understood the question?
>>>
>>> -Brian
>>
>



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