RFC1918 CIDR blocks no longer permitted when specifying Trusted IP ranges
Change effective on 04 December 2018
Previously, Heroku permitted RFC1918 CIDR ranges when specifying Trusted IPs for a Private Space. Using the Trusted IP feature to grant access to private network ranges never had any effect because the Trusted IP feature only blocks external connections from the public internet.
To avoid confusion, Heroku now blocks the use of RFC1918 CIDR ranges when specifying Trusted IPs.