I have been reading the RFCs for IPv6 addressing and I have stumbled across some IPv6 caveats that we need to consider when creating IPv6 DHCP pools and/or when creating netblocks that are larger than 64bits, actually larger than 70bits. This is not something that has not been discussed, but there is a part that does not make sense to me and I am hoping that someone has some experience that can definitively clear this up.
The items below effect the 18th digit, the 2nd digit in the 5th hexatet, in an IPv6 address.
- The inverted ‘u’ bit, also called the Universal/Local bit, is the 71st binary bit in an IPv6 address. I don't have a question about the 71st bit, but I am posting my understanding in case I have an incorrect understanding.
This bit indicates whether the address is universally administered, was assigned via stateless autoconfiguration, or is locally administered, assigned manually or via DHCP. Technically 0 means universally administered and 1 means locally administered, but the 71st bit is the inverse of that to make life easier for end users. Therefore, in an IPv6 address if the 71st bit is 0 the address is locally administered and the hexadecimal digit will be 0, 1, 4, 5, 8, 9, C or D and if it is 1 the address is universally administered and the hexadecimal digit will be 2, 3, 6, 7, A, B, E or F.
- The ‘u’ bit, also called the Individual/Group bit, is the 72st binary bit in an IPv6 address. This one I do not understand at all, it just does not make sense, so maybe I am having one of the following issues.
I have missed an address assignment in the RFCs.
I have failed to read between the lines.
I lack an understanding in how something works from a networking perspective, but the RFCs assume I have that knowledge and do not spell out the knowledge I lack.
The primary explanation I have for this bit comes from rfc5375 and I am not seeing anything in other RFCs that provide more information.
Based on the information I have the, 72nd bit indicates if the address is a unicast address or a multicast address. well, I do not see how that makes sense. I see that anycast addresses exist with-in unicast address space, but not multicast addresses as they all start with FFxx. The only thing I can find is that this may have something to do with MAC addresses. I found the following information, but I can't find anything about reserved MAC addresses? Couldn't a MAC address with a 1 in that position be on a legitimate non-multicast device?
Ethernet frames with a value of 1 in the least-significant bit of the first octet of the destination address are treated as multicast frames and are flooded to all points on the network. In an Ethernet frame, the least-significant bit of an octet is the first to be transmitted. A multicast is indicated by the first transmitted bit of the destination address being 1.