Subnet

A subnet is a logical subdivision of an IP network, defined by a prefix length (written as a CIDR suffix like /24) that marks how many of the address’s bits identify the network versus the individual host — the same prefix-length math Cloudflare’s explainer on subnets walks through. Subnetting lets an organization split one large address block into smaller, independently routable or administrable segments — separating a guest Wi-Fi network from internal servers, for example. The same math applies to deciding whether an address falls inside a reserved range, such as checking if a router’s WAN address sits inside the 100.64.0.0/10 block ISPs use for CGNAT — listed alongside the other reserved IPv4 blocks in IANA’s IPv4 Special-Purpose Address Registry — or inside the private ranges defined by RFC 1918. RFC 4632 is the specification behind the CIDR prefix notation subnetting relies on. The subnet’s first address (all host bits zero) and last address (all host bits one, the broadcast address) are both reserved rather than assignable to a device, which is why a /24 provides 254 usable addresses, not 256. Microsoft’s TCP/IP addressing and subnetting article works through the same example, using 192.168.123.1 to 192.168.123.254 for hosts because host portions of all zeros and all ones are invalid. To check whether one address matches a specific subnet — or several at once, such as a list of firewall allowlist entries — AdminKit’s IP in CIDR tool runs that containment test offline.