Best answer by ratty
The DHCP server configuration. You'll want to create mappings between MAC addresses and IP addresses. Usually the MAC addresses of the Sonos devices will already be in the DHCP client lease table. If not you can use the serial number, printed on the Sonos device itself or available via About My Sonos System, minus the last character and its preceding colon.
IP reservation in the DHCP server is the preferred way of fixing IP addresses. Use of static IPs, configured directly into the device itself, is error-prone and deprecated other than for certain fixed network devices such as the router itself. Sonos doesn't support the latter method.