Gateway Address Validator
Check an entered gateway against subnet rules.
Gateway Address Validator: inputs and results
Enter an IP address, CIDR prefix, or range. Then check its first and last address below.
Calculated Boundaries & ParametersReady
Key Capabilities
- •100% Client-Side: Zero telemetry or server latency.
- •RFC Compliant: Exact 32-bit and 128-bit integer precision.
- •Quick Copy: Instant access to CIDR, mask, and range bounds.
- •Cross-Platform: Works on desktop, tablet, and mobile browsers.
Related IPv4 Subnetting & RangeAll Tools
IPv4 Range Calculator
Find address boundaries from a prefix.
IPv4 Subnet Calculator
Calculate subnet details from address and mask.
Network Address Calculator
Find the subnet’s network address.
IPv4 Broadcast Address Calculator
Find broadcast addresses where applicable.
First and Last Address Calculator
Identify a range’s endpoints.
Usable IPv4 Host Range Calculator
Apply the selected subnet-use rules.
How Gateway Address Validator Works?
The Gateway Address Validator works by applying 4 rules to a proposed default gateway: same subnet as the host, not the network address, not the broadcast address, and not the host address itself.
Enter the host address and prefix
Type the host configuration, such as
10.40.8.25/22.Enter the gateway
Type the proposed gateway, such as
10.40.8.1.Compare the subnets
The validator computes the network of both addresses with the host mask and compares them.
Check reserved positions
The gateway fails when it equals the network address, the broadcast address, or the host address.
Gateway Address Validator Formula
The Gateway Address Validator formula is Valid = (GW AND Mask = Host AND Mask) AND GW ≠ Network AND GW ≠ Broadcast AND GW ≠ Host.
Same subnet
(GW AND Mask) = (Host AND Mask)The gateway must be on-link.
Not network
GW ≠ Host AND MaskApplies to /0 to /30.
Not broadcast
GW ≠ (Host AND Mask) OR WildcardApplies to /0 to /30.
Not self
GW ≠ HostA host cannot route through itself.
Worked example: host 10.40.8.25/22, gateway 10.40.11.254
Host network = 10.40.8.0 Broadcast = 10.40.11.255 Gateway network = 10.40.11.254 AND 255.255.252.0 = 10.40.8.0 ✓ Not network ✓ Not broadcast ✓ Not host ✓ Result = Valid
Gateway Address Validator Conversion Table
The Gateway Address Validator conversion table checks 10 host and gateway pairs and shows the reason for each result.
| Host | Gateway | Result |
|---|---|---|
| 10.40.8.25/22 | 10.40.8.1 | Valid |
| 10.40.8.25/22 | 10.40.11.254 | Valid |
| 10.40.8.25/22 | 10.40.12.1 | Invalid: outside host subnet |
| 192.168.1.50/24 | 192.168.1.0 | Invalid: network address |
| 192.168.1.50/24 | 192.168.1.255 | Invalid: broadcast address |
| 192.168.1.50/24 | 192.168.1.50 | Invalid: same as host |
| 192.168.1.50/25 | 192.168.1.129 | Invalid: outside host subnet |
| 172.16.0.10/30 | 172.16.0.9 | Valid |
| 10.255.0.0/31 | 10.255.0.1 | Valid |
| 203.0.113.70/27 | 203.0.113.65 | Valid |
What is Gateway Address Validator?
The Gateway Address Validator is a tool that checks whether a proposed default gateway is a reachable, assignable address in the same subnet as the host.
A host sends off-subnet traffic to its default gateway. The gateway must sit on the same subnet as the host, or the host cannot reach it with Address Resolution Protocol (ARP). A gateway set to the network or broadcast address fails in the same way.
Misconfigured gateways are a frequent cause of "local works, internet fails" tickets. The validator finds the error from the configuration alone, before anyone touches the device.
What are the uses of Gateway Address Validator?
There are 4 main uses of the Gateway Address Validator, including static IP checks, DHCP option checks, and troubleshooting.
Checking static IP configurations
Technicians validate the address, mask, and gateway before saving a static configuration.
Reviewing DHCP router options
DHCP option 3 (router) must match the scope subnet. The validator checks each scope.
Troubleshooting off-net failures
A host that reaches local devices but not the internet often has a gateway outside its subnet.
Auditing configuration exports
Scripts run the 4 rules on every device in an inventory export.