After receiving the DHCP-ACK message, the client probes whether the IP address assigned by the
server is in use by broadcasting a gratuitous ARP packet. If the client receives no response within
a specified time, the client can use this IP address. Otherwise, the client sends a DHCP-DECLINE
message to the server and requests an IP address again.
The IP addresses offered by other DHCP servers are still assignable to other clients.
IP Address Lease Extension
The IP address dynamically allocated by a DHCP server to a client has a lease. When the lease expires,
the IP address is reclaimed by the DHCP server. If the client wants to use the IP address longer, it has
to extend the lease duration.
When the half lease duration elapses, the DHCP client sends to the DHCP server a DHCP-REQUEST
unicast to extend the lease duration. Upon availability of the IP address, the DHCP server returns a
DHCP-ACK unicast confirming that the client's lease duration has been extended, or a DHCP-NAK
unicast denying the request.
If the client receives no reply, it broadcasts another DHCP-REQUEST message for lease extension
after 7/8 lease duration elapses. The DHCP server handles the request as above mentioned.
DHCP Message Format
gives the DHCP message format, which is based on the BOOTP message format and
involves eight types. These types of messages have the same format except that some fields have
different values. The numbers in parentheses indicate the size of each field in bytes.
Figure 1-3 DHCP message format
op: Message type defined in option field. 1 = REQUEST, 2 = REPLY
htype, hlen: Hardware address type and length of a DHCP client.
hops: Number of relay agents a request message traveled.
xid: Transaction ID, a random number chosen by the client to identify an IP address allocation.