RFC 1918 Netmask

When using RFC1918 (IPv4) for 20-bit block private address space
(≈1 million addresses), special caution is needed.

For 172.16.0.0/12 networks, the correct default netmask is 255.240.0.0

Setting a netmask other than 255.240.0.0 will unintentionally block out a big chunk of 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 = 1,048,576