This article applies to all Aruba Mobility Controllers and ArubaOS
It is not recommended to use unequal subnet masks for vlans clubbed together in a vlan pool as this causes issues with the vlan pooling algorithms implemented on the Controller whether through hashing algorithm or "even" dhcp pool. When the wireless clients are assigned IP addresses in vlans selected from the vlan pool, the basic assumption by the Controller is that all vlans in the pool are equal sized subnets.
The need for load balancing between two vlans based on their DHCP scope usage, particularly unequal sized vlans, cannot be achieved using vlan pooling. The base reason for this is that the Controller can keep track of the number of clients assigned to each vlan in the pool but cannot track their lease times. In other words, the Controller does not maintain or check whether the DHCP scope for a vlan is full or not, before assigning a client to the vlan.