DHCP option 66 is coming from the past when tftp was used for provisioning, today the option is also used to provide the HTTP(S) url.
As the Gigaset DHCP provisioning URL option is 114 instead of 66, we have implemented also the option 66. Option 114 has higher priority and will overwrite option 66. See the wiki article for more information.
The phone requests the provisioning server address via a DHCP request with option 66 (dhcp_url). This method is predominantly used by stand-alone provisioning servers within the same LAN.