I see several discussions about the 50 IP address limit but not an answer about what counts towards the 50.
From the wording in the warning about approaching the limit, I thought it was IPs being routed/NATted through the UTM. I configured several devices to not access the internet, with no change in my IP usage. I pointed them also to a separate DNS server that only answers local queries. No change after that.
The next thing I was going to change was DHCP. Do IPs getting DHCP leases count towards the 50? If so, is there any service I've left out?