IP Default Subnet Masks For Address Classes A, B and C

URL : http://www.tcpipguide.com/free/t_IPDefaultSubnetMasksForAddressClassesABandC.htm

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255.255.0.0:,This is the default mask for Class B, but can also be the custom subnet mask for dividing a Class A network using 8 bits for the subnet ID (leaving 16 bits for the host ID).

255.255.255.0: This is the default subnet mask for Class C, but can be a custom Class A with 16 bits for the subnet ID or a Class B with 8 bits for the subnet ID.