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DHCP fails if IP Pool overlaps

Even an unused IP Pool definition can stop DHCP from working if the IP Pool & DHCP range overlap
Chris Hall
Fortinet Technical Support

Hi Chris,


Yes, that's correct. If an IP pool is defined but not used, it can still cause issues with DHCP if the IP pool's range overlaps with the DHCP range. This situation can lead to conflicts and unpredictable behavior in the network.

When a DHCP server assigns IP addresses to devices, it must ensure that the IP addresses fall within a specific range, known as the DHCP range. If there is an overlap between the IP pool range and the DHCP range, it can result in two main problems:
1.IP Address Conflicts
2.Unpredictable DHCP Behavior

Babitha M

Contributor III

Yes, Chris, you're correct. Overlapping IP Pools and DHCP ranges can cause issues in a network, even if the IP Pool is not actively being used. This is a common pitfall in network configuration and something that network administrators need to be cautious about. Here's why this can be a problem and what you might do to avoid it:

### Why Overlapping IP Pools and DHCP Ranges Cause Problems
- **Conflicting Assignments**: If an IP address is part of both an IP Pool and a DHCP range, the system may become confused about how to handle that address. It can't simultaneously reserve it for the IP Pool and offer it as part of the DHCP range.
- **Potential for Duplicate IPs**: If the DHCP server is unaware of the IPs reserved in the IP Pool, it might assign an IP to a client that's already been assigned elsewhere, leading to an IP conflict.

### How to Avoid These Issues
1. **Carefully Plan IP Ranges**: When defining IP Pools and DHCP ranges, careful planning is essential to ensure that the ranges do not overlap. Documenting these ranges can help avoid confusion later.
2. **Use Subnets**: Consider dividing your network into subnets to segregate different types of traffic and assignments. You might have one subnet for DHCP clients and another for statically assigned IPs or IP Pools.
3. **Utilize Monitoring and Management Tools**: Network monitoring tools can help you keep track of IP assignments and alert you to potential conflicts.
4. **Regularly Review Configuration**: Periodically review and audit your network configuration to ensure that everything is set up correctly and that there are no latent issues like overlapping ranges.
5. **Consider DHCP Reservations**: If you need to reserve specific IPs within a DHCP range, consider using DHCP reservations rather than separate IP Pools. This allows the DHCP server to manage all the IPs and ensures that there are no conflicts.

### Conclusion
Overlapping IP Pools and DHCP ranges can indeed lead to confusing and hard-to-diagnose issues. Careful planning, clear documentation, and regular review of your network configuration can help avoid these problems. If you find an overlapping range in your network, resolving it by adjusting the IP Pool or DHCP range to eliminate the overlap should clear up the issue.


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