subnet tools
Subnet Calculator
The Subnet Calculator turns an IP address and prefix length into the network details engineers usually need during planning or troubleshooting. It supports IPv4 ranges and IPv6 prefix calculations.
IPv4 Results
- Network address, broadcast address, first usable host, and last usable host.
- Subnet mask, wildcard mask, CIDR prefix, and total address count.
- Class hints for older documentation or support references.
IPv6 Results
- Prefix-normalized network start and end addresses.
- Total address count for the selected prefix length.
- Quick confirmation that an entered address belongs to the expected prefix.
When To Use It
Use this tool when building firewall rules, assigning small customer LANs, checking DHCP scopes, splitting address space, or translating between CIDR and dotted-decimal notation.
How To Read Results
- Network address is the start of the subnet and is usually not assigned to a host.
- Broadcast address is the last address in most IPv4 subnets and is not used by regular hosts.
- Usable range is the normal host assignment range between network and broadcast.
- Wildcard mask is commonly used in ACLs and is the inverse of the subnet mask.
- For /31 and /32 IPv4 networks, point-to-point and host-route behavior can differ from older usable-host rules.
How To Use Subnet Calculator
- Enter an IPv4 address and CIDR prefix such as 192.168.1.10/24.
- Review the network, broadcast, usable host range, subnet mask, and wildcard mask.
- Use the usable range for DHCP or host planning, and the network address for routes and firewall objects.
- For IPv6, enter the address and prefix to confirm the normalized prefix range.
Troubleshooting Flow
Example Result
Network: 192.168.1.0
Usable hosts: 192.168.1.1 - 192.168.1.254
Broadcast: 192.168.1.255
Subnet mask: 255.255.255.0
- The entered host belongs inside the 192.168.1.0/24 network.
- Most IPv4 LANs reserve the first and last address.
- Use the wildcard mask when translating to some ACL formats.
Network: 10.20.30.0
Usable hosts: 10.20.30.1 - 10.20.30.14
Next /28 starts: 10.20.30.16
- A gateway or static host outside the usable range will not behave like a local peer.
- Small prefixes leave less room for DHCP reservations.
- Check route and firewall objects against the same prefix length.
What Good And Bad Results Look Like
- The host IP falls inside the expected CIDR range.
- Firewall and DHCP scopes use the same network boundary.
- The usable host range matches the plan.
- The gateway sits outside the calculated subnet.
- Two VLANs overlap unintentionally.
- A route object uses the wrong prefix length.
Common Mistakes
- Using the network or broadcast address as a normal host address on traditional IPv4 LANs.
- Mixing up subnet mask and wildcard mask in firewall or ACL work.
- Forgetting that /31 and /32 networks follow special host-route or point-to-point rules.
Practical Troubleshooting Workflow
- Enter the host IP and CIDR from the design or ticket.
- Confirm network and broadcast boundaries.
- Check usable host range before assigning addresses.
- Use the wildcard output only where the platform expects wildcard notation.
FAQ
What does /24 mean in a subnet?
/24 means the first 24 bits are the network portion. In IPv4, that usually corresponds to the subnet mask 255.255.255.0 and 256 total addresses.
Why are usable hosts lower than total addresses?
Traditional IPv4 subnets reserve the network address and broadcast address, so the normal usable host count is two fewer than the total address count.
Do IPv6 subnets have broadcast addresses?
No. IPv6 does not use broadcast the way IPv4 does, so IPv6 subnet planning focuses on prefixes and address ranges instead of broadcast addresses.