Subnet mask
A bitwise mask that splits an IPv4 address into a network part and a host part. 255.255.255.0 = /24 = 24 network bits + 8 host bits = 256 addresses. Modern usage always writes it as a CIDR prefix (/24) instead of dotted decimal.
AND between IP and mask gives the network address. /30 (4 addresses, 2 usable) is typical for point-to-point links; /24 typical for a home LAN; /16 for large corporate networks. Classful networking (Class A/B/C) is historical — CIDR (1993) replaced it. VLSM (Variable Length Subnet Mask) = different subnets have different prefix lengths, normal today. IPv6 has 64-bit prefixes as standard for end networks, /64 = ~1.8e19 addresses per LAN. Subnet design is the classic junior network engineer test.