A netmask that contains 1-bits after 0-bits (the 1-bits are not contiguous on the left side) is invalid  . The code before this PR used to parse and accept such non-left-contiguous netmasks. However, a coming change that will alter `CNetAddr::ip` to have flexible size would make juggling with such netmasks more difficult, thus drop support for those.  https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing#Subnet_masks  https://tools.ietf.org/html/rfc4632#section-5.1
Partial backport (1/2) of core PR19628:
Depends on D9172.