Why flip the 'u' bit when creating the EUI-64 ID?

Unanswered Question
Sep 29th, 2007
User Badges:
  • Cisco Employee,

Hello anyone,

I've done some study of the RFC 4291 where the IPv6 addressing is required. There are two issues that puzzle me:

1.) When creating the 64-bit Interface ID from the MAC address or from the original EUI-64, the U/L bit (the 7th bit of the topmost byte) is inverted. Why is this done? The RFC tries to explain it in a few words but I do not find the explanation sufficient.

2.) If I am specifying the Interface ID of the IPv6 address manually, should I take care that the U/L bit is set to a specific value (in this case, 0)?

Any hints are heartily welcome!

Best regards,


  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)


This Discussion