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1 subnet mask
"A 32-bit value that enables the recipient of Internet Protocol version 4 (IPv4) packets to distinguish the network ID and host ID portions of the IPv4 address. Typically, subnet masks use the format 255.x.x.x. IPv6 uses network prefix notations rather than subnet masks." -
2 default subnet mask
A subnet mask that is used on an Internet Protocol version 4 (IPv4) class-based network. The subnet mask for Class A is 255.0.0.0. The subnet mask for Class B is 255.255.0.0. The subnet mask for Class C is 255.255.255.0.