v4 side unmodifiable

[Iljitsch van Beijnum <iljitsch@muada.com>]

Jari basically said this ^^

However, although I agree it's true in the general case, I think for  
non-trivial servers/services, this isn't true: if there is a good  
reason to make changes on the IPv4-only side, it may be possible to  
make them.