What CIDR will block the entire 172.x.x.x private networks?

Resolution

According to RFC 1918, there are 16 contiguous Class B networks reserved for private use:

172.16.0.0 - 172.31.255.255

If such private networks have to be matched by a single CIDR, for example for blocking etc, the following expression should be used:

172.16.0.0/12