IPv6Prefix incorrectly handles IPv6 with prefix non-divisible by 8

[dav-komarek]

Follow up to #118

The fix in the linked issue doesn't seem to be correct. The fix incorrectly handles IP addresses with the prefix non-divisible by 8.

For details see 1006025#r134862652

[dav-komarek]

Any update here? Should I propose PR?

[u873838]

Should I propose PR?

That would be appreciated, thank you.