Troubleshooting Tip: Resolving DHCP Pool Exhaustion on FortiGates

To resolve the issue of some PCs being unable to obtain a DHCP IP address, follow these steps:

1. Go to Network -> Interface to check the current DHCP settings.
2. Verify that the DHCP IP pool is not exhausted by checking the number of available IPs in the specified range.

3. If the IP pool is exhausted, select Edit to expand the existing DHCP IP pool by adding a new range.

For example:

Given that the subnet 10.0.0.0/21 supports 2,048 hosts, the issue was addressed by expanding the existing DHCP IP pool. The previously defined ranges were:

• 10.0.1.100 – 10.0.2.254.
• 10.0.3.100 – 10.0.3.254.
• 10.0.4.100 – 10.0.4.254.

To make more IP addresses available, a new range was added:

• 10.0.5.100 – 10.0.5.254 (providing an additional 253 IPs).

4. Verify that newly connected PCs are now able to successfully receive IP addresses.

Related article:

Technical Tip: Diagnosing DHCP on a FortiGate