At shutdown, the client sends a Release message to the server that assigned addresses to the client to indicate that the client will no longer use one or more of the assigned addresses. When the DHCPv4 client system shuts down normally, dhcpagent writes the current configuration information to a file, if the file exists. The filename for DHCPv4 is /etc/dhcp/interface.dhc, and /etc/dhcp/interface.dh6 is for DHCPv6. Because the lease is saved rather than released by default, the DHCP server cannot detect that the IP address is not in active use, which enables the client to easily regain the address on next boot. This default action is the same as the ipadm delete-addr DHCP-addrobj command.
If the lease in that file is still valid when the system reboots, dhcpagent sends an abbreviated Request (DHCPv4) or Confirm (DHCPv6) message to use the same IP address and network configuration information.
If the DHCP server permits this request, dhcpagent can use the information that it wrote to disk when the system shut down. If the server does not permit the DHCP client to use the information, dhcpagent initiates the DHCP protocol sequence described in How DHCP Works. As a result, the client obtains new network configuration information.