Subnet Calculator

Subnetting basics (IPv4)

Subnetting splits a larger IP network into smaller segments (subnets) so you can organize hosts, contain broadcasts, and apply routing or security policies per segment. In CIDR notation (e.g., 192.168.1.10/24), the prefix length (/24) tells you how many of the first bits are the network portion. The rest are for host addresses inside that subnet.

• Subnet mask: dotted-decimal form of the prefix (e.g., /24 → 255.255.255.0).
• Wildcard mask: the inverse of the subnet mask (often used in ACLs). Example: 255.255.255.0 → 0.0.0.255.
• Network address: all host bits set to 0 (the first address of the subnet).
• Broadcast address: all host bits set to 1 (the last address of the subnet).
• Usable host range: addresses between network and broadcast (classic IPv4). For /31 and /32, see notes below.

How this calculator works

Given IP and /prefix:

mask        = first  bits set to 1 (32-bit)
wildcard    = bitwise NOT of mask
network     = IP AND mask
broadcast   = network OR wildcard
total hosts = 2^(32 - prefix)
usable      = (prefix <= 30) ? (total - 2) : 0
host range  = (prefix <= 30) ? [network+1 ... broadcast-1] : N/A
      

Note on /31 and /32: in “classic” accounting, there are no usable hosts because network/broadcast take the edges. In practice, /31 is commonly used on point-to-point links (RFC 3021) where both addresses are assigned and no broadcast exists; /32 represents a single host route.

Quick examples

Example 1: 192.168.1.10/24

• Mask: 255.255.255.0 · Wildcard: 0.0.0.255
• Network: 192.168.1.0 · Broadcast: 192.168.1.255
• Usable range: 192.168.1.1 – 192.168.1.254 (254 hosts)

Example 2: 10.12.34.56/20

• Mask: 255.255.240.0 · Wildcard: 0.0.15.255
• Network: 10.12.16.0 · Broadcast: 10.12.31.255
• Usable range: 10.12.16.1 – 10.12.31.254 (4094 hosts)

Private IPv4 ranges (for LANs)

• 10.0.0.0/8   (10.0.0.0 – 10.255.255.255)
• 172.16.0.0/12 (172.16.0.0 – 172.31.255.255)
• 192.168.0.0/16 (192.168.0.0 – 192.168.255.255)

FYI: 169.254.0.0/16 is link-local (APIPA)—assigned automatically when DHCP fails; not routed.

Common CIDR cheat sheet