Subnet Mask
A value that separates an IP address into its network and host portions.
A subnet mask is a 32-bit value written in dotted-decimal notation (such as 255.255.255.0) that identifies which bits of an IP address belong to the network and which belong to the host. A device performs a bitwise AND of the mask against a destination IP to extract the network address and decide whether traffic is local or must be routed. On N10-009, know that a subnet mask and a CIDR prefix carry identical information expressed differently: 255.255.255.0 equals /24. Borrowing host bits creates more subnets but fewer hosts; subtract 2 per subnet for the network and broadcast addresses.
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