IPv4 subnet calculator

Every IPv4 network is defined by an address and a prefix length, such as 192.168.10.0/26. From that, you can derive the subnet mask, the network and broadcast addresses, the range of usable host addresses and how many devices fit. This calculator does all of that instantly, which is essential for planning networks, configuring routers and writing firewall rules.

How to use it

1. Enter an address with a CIDR prefix, for example 192.168.10.37/26.
2. Read the network details.

Worked example: 192.168.10.37/26

• Subnet mask: 255.255.255.192 (26 ones followed by 6 zeros)
• Wildcard mask: 0.0.0.63, used in Cisco ACLs and OSPF
• Network address: 192.168.10.0
• Broadcast address: 192.168.10.63
• Usable hosts: 192.168.10.1 to 192.168.10.62, which is 62 hosts

The math

Host bits = 32 - prefix = 6. Addresses per subnet = 2^6 = 64. Usable hosts = 64 - 2, because the network and broadcast addresses cannot be assigned to devices. The network address is found by ANDing the IP with the mask: 37 AND 192 = 0.

Quick reference

• /24: 256 addresses, 254 hosts
• /25: 128 addresses, 126 hosts
• /26: 64 addresses, 62 hosts
• /27: 32 addresses, 30 hosts
• /28: 16 addresses, 14 hosts
• /30: 4 addresses, 2 hosts, classic for point-to-point links
• /31: 2 addresses, both usable on point-to-point links (RFC 3021)

Cloud note

AWS, Azure and Google Cloud reserve extra addresses in every subnet (AWS reserves 5), so a /28 in AWS gives 11 usable addresses, not 14.