When using RFC1918 (IPv4) 20-bit block private address space, special caution is needed

For 172.16.0.0/12 networks , the correct default netmask should be  255.240.0.0 
binary = 11111111 11110000 00000000 00000000  

Using a netmask other than 255.240.0.0 [172.16.0.0/12 CIDR block] may unintentionally mask out perfectly valid IP addresses.

For the 172.16.0.0/12 networks:
Base address 172.16.0.0
Highest address 172.31.255.255
Number of IPv4 addresses is 1,048,576