Compile locally and install
sudo gcc cidr.c -o /usr/local/bin/cidr
See https://www.freebsd.org/cgi/man.cgi?query=cidr&manpath=FreeBSD+12.2-RELEASE+and+Ports for more details
A special thanks to Robert L. Lineberger, David A. Bandel, Iain Lea, and Herman Robers for 20+ years of elegance and the entire FreeBSD team.