Subnet Calculator

CIDR Subnet Calculator Online

101IP calculates IP subnet parameters from CIDR notation. Enter an address with a mask to get the network address, broadcast, host range, subnet mask and total address count.

What the calculator returns

• Network address — first address of the subnet;
• Broadcast — last address in the range;
• Host range — first and last usable host addresses;
• Subnet mask — dotted-decimal and CIDR prefix;
• Address count — total and usable host count.

Frequently Asked Questions

• How do I convert CIDR notation to a subnet mask?

  Enter the IP address with its prefix, for example 192.168.1.0/24. The calculator will automatically show the subnet mask 255.255.255.0. You can also enter the mask directly — the tool will determine the corresponding CIDR prefix.

• Why does a /31 subnet show 0 usable hosts?

  A /31 mask leaves only 1 bit for the host portion, giving exactly 2 addresses (network and broadcast). Normal subnets require at least 2 usable host addresses, so /31 is used only in special point-to-point configurations (RFC 3021). The calculator displays this correctly.

• How can I check whether an IP address belongs to a specific subnet?

  Enter the address in the calculator and then adjust the mask to the target subnet. The tool will show the network address and host range. If your IP falls between the first and last host address, it belongs to that subnet.