You can create static mappings between client and IP address for some or all clients. Starting with Sophos UTM version 9.1, this feature has moved to the Definitions & Users > Network Definitions tab. DHCP mappings are now defined along with the involved hosts.
When you click the Static Mappings button, the Definitions & Users > Network Definitions tab opens. Automatically, only hosts with static mapping are displayed. Use the drop-down list on top of the list to change the filter settings.