In D19316#718678, @karels wrote:

I haven't come up with a sysctl name that I really like.

Two comments: RFC 3021 eliminates both the all-ones broadcast address and the all-zeroes broadcast address on an interface when configured as a /31. Is that all-ones address already handled somehow by how ia->ia_broadaddr.sin_addr.s_addr is getting set elsewhere? In both the old and new code, the IN_RFC3021_MASK test is not eliminating the all-ones broadcast address in this function.

Hello again to @rgrimes and Mike @karels!

In my first pass audit of a search results on IN_CLASS[A-E] the common use cases here are the default calculation of netmasks, 2 minutes ago I would of told you that probably can not go away, but it just hit me, IN_DEFAULTMASKFOR(i) could be written that would incorporate the ancient class rules in a central place and cleaning up some code. Thoughts?

Thanks @rgrimes! Hi Mike @karels! John Gilmore here...