Solution:

There is rule between IP and Subnet mask. For the first word of IP address (xxx.xxx.xxx.xxx) If it is in between 1~126, the subnet Mask has to be 255.xxx.xxx.xxx Between 128~191 the subnet Mask has to be 255.255.xxx.xxx Between 192~, the subnet Mask has to be 255.255.255.xxx For example, if the IP address is 192.24.168.2, the first word of IP address is 192. Therefore the subnet Mask should be 255.255.255.0