In agent mode, when machines are discovered initially, we will map unique ID for specific machine and rest of the communication from that agent will be using that unique ID. Hence even if IP changes, as unique ID remains same, it will show as single entry in SapphireIMS.
In agentless mode, we rely upon multiple attributes like system serial number, MAC ID, hostname, IP address and OS type to figure the node is unique or not. SapphireIMS is capable to discover machine even in DHCP network. SapphreIMS identifies DHCP enabled devices during node discovery and even has option to mark devices as DHCP enabled using DHCP scope profile as part of discovery configuration. Once discovery is done, for subsequent communication with device, SapphireIMS use hostname instead of IP address so that we use latest resolved IP Address. Even we have to option to refresh the discovered IP addresses by fetching ARP table entries ( Using MAC-IP Pair we update IP addresses in SapphireIMS records where specific MAC matches ) from gateways configured using SNMP and refreshing local ARP cache on frequent basis.