Currently, it's doing DHCP option 82 to relay the DHCP requests from the management ports on modems, through the DSLAMs, into the VM. The management ports have defined IP addresses per card\port combination.
They're all on a shared network. I have all of the DSLAMs specified as hosts.
Webmin documentation says that all subnets on the same LAN segment must be in the same shared network and a shared network must only contain subnets that are on the same LAN.
Currently, that's so. However, adding more VLANs and subnets... they aren't in the same LAN anymore. However, the DSLAMs (currently) are.
I assume this is less about Webmin and more about ISC-DHCP.
Unfortunately, I don't remember much of when I set up that DHCP server to remember the nuances of ISC DHCP and option 82.