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

Brian Haley brian.haley at hp.com
Wed Feb 13 02:48:33 GMT 2013


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.

-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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