IPv4 mask the legitimacy of research

The following types of illegal Mask Gateway:

1. Class A IP address 0 == IP & SubnetMask subnet mask illegal
2. SubnetMask class B IP addresses in the number of 1 <= 1 is not legal subnet mask
3. SubnetMask Class C IP addresses in the number of 1 <= 2 is not legal subnet mask
4. Class D, IP address of the E class are not allowed (i.e. a range of IP addresses is not equal to [1,233] 127)