Explanation

DHCPACK
The server-to-client communication with configuration parameters, including committed network address.
DHCPINFORM
The client-to-server communication, asking for only local configuration parameters that the client already has externally configured as an address.
DHCPNAK
The server-to-client communication, refusing the request for configuration parameter.
DHCPDECLINE
The client-to-server communication, indicating that the network address is already in use