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Calculate subnet masks, network ranges, and usable hosts from IP addresses and CIDR notation.
A subnet (subnetwork) is a logical division of an IP network that groups devices together, defined by a subnet mask that determines which portion of an IP address identifies the network vs. individual hosts.
| CIDR | Mask | Hosts |
|---|---|---|
| /30 | 255.255.255.252 | 2 |
| /28 | 255.255.255.240 | 14 |
| /24 | 255.255.255.0 | 254 |
| /16 | 255.255.0.0 | 65,534 |
| /8 | 255.0.0.0 | 16,777,214 |
Network Address: Bitwise AND of IP address and subnet mask.
Host Range: First host = network + 1, last host = broadcast - 1 (except /31 and /32).
Usable Hosts: 2^(32 - CIDR) - 2
Formula
Network Address = IP AND Subnet Mask | Usable Hosts = 2^(32 − CIDR) − 2 | Broadcast = Network OR Wildcard MaskCIDR = number of bits used for the network portion (e.g., /24 = 24 network bits)
Subnet Mask = binary mask where network bits are 1 and host bits are 0
Wildcard Mask = inverse of the subnet mask — used for broadcast and ACL rules
− 2 = subtracts the network address and broadcast address, which cannot be assigned to hosts
Worked Example
IP: 192.168.1.100, CIDR: /24
Did you know? The /24 subnet (255.255.255.0) is the most common in home and small office networks — it provides 254 usable addresses and maps neatly to one octet. RFC 1918 reserves three private ranges so internal networks never conflict with public internet addresses.
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