change to deprecate broadcast on host 0 of a subnet

From: Mike Karels <>
Date: Sun, 12 Sep 2021 15:25:56 UTC
Long ago (4.2BSD), the IP broadcast address was the lowest address on a
network, the one with a host part of 0.  In RFC1122, the broadcast address
was standardized using a host part of all ones.  4.3BSD changed its
default, and made the broadcast address settable with ifconfig.  However,
FreeBSD *still* broadcasts packets sent to the lowest address on a subnet.

I have a change in review to stop broadcasting the lowest address on a
subnet by default, but added a sysctl to revert to the current behavior.
I really doubt that anyone is still using a 0-based broadcast address.
This change allows host 0 on a subnet to be used as an assigned host
address, as long as the systems on that network support it (including
routers).  Linux already has this change.

The review is  See also
https:/ and
some of the discussion in

Comments are welcome on the review.  I will wait a couple of days
for comments before proceeding.  I am also interested in comments on
whether this should be MFC'ed to 13-stable after a suitable delay.