DHCPNAK
DHCPNAK stands for DHCP Negative Acknowledgement. It is a DHCP message, type 6, sent by a DHCP server to indicate that a client's request for an IP address or configuration cannot be granted. It is used in the DHCP exchange after a client has sent a DHCPREQUEST, typically when the requested IP address is unavailable, not valid for the client's subnet, or no longer matches the server's records (for example if the lease has expired or the client identifier does not match the server's data). A DHCPNAK signals that the current parameters cannot be used and the client must start the process over.
In normal operation, DHCP clients proceed through a four-step process: DISCOVER, OFFER, REQUEST, ACK. If the REQUEST
On receiving a DHCPNAK, the client should abandon its current IP configuration, return to the INIT state,