IPv6 Address Planning

Thu Aug 11 07:44:44 UTC 2005

On 11-aug-2005, at 2:23, Kevin Loch wrote:

>> And on that vein perhaps it's prudent for people using network
>> prefixes longer than /64 to take care to ensure that the bit  
>> positions
>> in the IPv6 address that should correspond to the u and g bits in the
>> modified EUI-64 interface ID (according to RFC 3513) are both set to

> Is there any known use for those bits?

The universal/local bit is copied from the EUI-64/MAC address and  
flipped, and indicates whether the address is derived from something  
(supposedly) globally unique or not. Both occur frequently, non- 
unique stem from manual configuration or RFC 3041 temporary/privacy  
addresses. The group bit isn't relevant, although you won't see MAC- 
derived addresses with this bit set, of course.

There is no real reason to preserve these bits when the prefix length  
is > 64.