Can't configure local Fortigate IPAM with Class C

[HossamAdel]

Hello,

When trying to create in Fortigate an IPAM subnet with a Class C subnet (192.168.0.0/24), I received an error stating that the mask must be within the range 8–16.

The document below also states that the IPAM pool subnet can only be a Class A or Class B subnet.

I’m trying to understand the reason for this limitation, or whether I might be missing something, since our current subnet is Class C.

https://docs.fortinet.com/document/fortigate/7.6.2/administration-guide/166728/configure-ipam-locall...

[xshkurti · Accepted solution]

@HossamAdel 

Given the design and usage of IPAM, there are several implied reasons why Class C pools are disallowed:

• Insufficient size: A /24 (class-C) network gives only 256 IP addresses.
  IPAM’s purpose is to allocate multiple subnets (potentially of varying sizes), and a /24 subnet does not fit into this design

• Subnet allocation logic: IPAM logic expects enough address space to give out multiple sub-subnets (child networks). If the “pool” is too small, you might see overlapping subnets and other issues merging/spliting subnets etc.
• Scalability and future growth: A larger pool (Class A or B) gives headroom for network growth. 

For only /24 subnet, you can use normal DHCP under network interface.

If your requirement is to operate only a /24 network under IPAM, but the software insists on a larger pool, use a larger supernet (e.g. /16 ) as the pool, then manually assign /24 (or /25, /26 as needed) sub-nets to interface of your choice.