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

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

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



For the 172.16.0.0/12 networks
Base address
172.16.0.0  binary 10101100  00010000  00000000  00000000

Highest address
172.31.255.255  binary 10101100 00011111 11111111 11111111

Number of IPv4 addresses in this private subnet block is 1,048,576


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