I have a doubt. IP addresses of the PCs connected to this network do not seem to change every time they connect to the network. I should tidy up the IP addresses, respecting the physical layout of the PC. The firewall is a fortigate 90d, while the firmware version is v5.0
@Nroot: your explanation is not quite right. The FGT doesn't reserve an address for a specific MAC address unless manually configured to do so. Rather, the client requests the same address as previously obtained. And shortening the lease interval will just create more traffic, not more addresses - for the same reason.
If you want to control the address a host uses either assign static addresses or set up reserved DHCP addresses on the FGT.
Please explain what do you mean by saying "tidy up", and "respecting the physical layout of the PC"? And it's not a bad thing to have your pc get same ip add eventhou it's getting it from dynamic dhcp pool. It's just mean that in dhcp binding table of the fortinet your pc mac address is associated with that offered ip add. You can make dhcp changing more frequently on pc if you decrease dhcp lease time on fortinet dhcp pool configuration. I hope this helps