Explain subnet masks and CIDR notation.

Subnet mask defines network vs host portions of IP address. Binary: 1s for network, 0s for host. Example: 255.255.255.0 means first 24 bits are network. CIDR notation: IP/prefix-length (192.168.1.0/24). Subnetting divides networks into smaller segments. /24 = 256 addresses (254 usable), /16 = 65,536 addresses. Calculate: usable hosts = 2^(32-prefix) - 2 (network and broadcast addresses). VLSM (Variable Length Subnet Masking) allows different-sized subnets.