IP Subnet Calculator

Examples

The results below come from the calculator’s own functions.

The first row is the usual question when you see an address in a log: 192.168.1.77 with a /26 mask belongs to the block that starts at .64, so 192.168.1.130 is in a different subnet and traffic between them goes through a router. The CIDR Notation field shows the corrected form, 192.168.1.64/26, which is what most firewall and cloud consoles expect; many of them reject 192.168.1.77/26 because host bits are set.

Copy writes the eight fields as labelled lines, ready for a ticket or a change request:

Network Address: 192.168.1.64
Broadcast Address: 192.168.1.127
Subnet Mask: 255.255.255.192
Wildcard Mask: 0.0.0.63
First Usable Host: 192.168.1.65
Last Usable Host: 192.168.1.126
Usable Hosts: 62
CIDR Notation: 192.168.1.64/26

The second row is a typical cloud subnet. The calculator counts 4,094 usable hosts, but AWS reserves the first four and the last address of every VPC subnet, and Azure reserves five as well, so only 4,091 can be assigned there. Subtract those yourself when you size a cloud subnet.

For /31 and /32 the calculator does not subtract a network and a broadcast address. A /31 is a point-to-point link with two usable addresses (RFC 3021), and a /32 is a single host, as used in route tables and allow-lists. Neither has a broadcast address, so the Broadcast Address field says None and gives the reason:

Broadcast Address: None (/31 point-to-point link, RFC 3021)
First Usable Host: 203.0.113.8
Last Usable Host: 203.0.113.9
Usable Hosts: 2

Output Fields Explained

Common Subnet Reference

Errors and Limitations

• Addresses must be four decimal numbers from 0 to 255 with no leading zeros. 10.0.0.01, 0x0a.0.0.1 and 10.1 show “Invalid IPv4 address. Use four numbers from 0 to 255 separated by dots, with no leading zeros (e.g. 10.0.0.1).” This matches inet_pton and Node’s net.isIPv4; it avoids guessing whether 010 means 10 or octal 8.
• Before that check, full-width digits, dots and slash (１９２．１６８．１．７７／２６) and the ideographic full stop (192。168。1。77/26) that an IME may type are read as ASCII. The text in the field is left as you typed it.
• The prefix must be a whole number from 0 to 32. /33, /24x and /024 show “The CIDR prefix must be a whole number from 0 to 32 (e.g. /24).”
• Dotted masks are not accepted after the slash. Type /26, or look up the prefix for a mask in the table above.
• IPv4 only. IPv6 addresses are rejected as invalid.
• One subnet at a time: the tool does not split a block into smaller subnets, list every address, or check whether two ranges overlap.

Common Use Cases

• Network design and VLAN planning
• Verifying firewall and ACL rules
• Troubleshooting routing issues
• Cloud VPC and security-group configuration (AWS, GCP, Azure)
• OSPF and BGP prefix calculations