The DHCP server issues a passive open command on UDP port number 67 and
waits for a client.
2. A booted client issues an active open command on port number 68 (this number
will be explained later). The message is encapsulated in a UDP user datagram,
using the destination port number 67 and the source port number 68. The UDP user
datagram, in turn, is encapsulated in an IP datagram. The reader may ask how a client
can send an IP datagram when it knows neither its own IP address (the source
address) nor the server’s IP address (the destination address). The client uses all 0s
as the source address and all 1s as the destination address.
3. The server responds with either a broadcast or a unicast message using UDP
source port number 67 and destination port number 68. The response can be unicast
because the server knows the IP address of the client. It also knows the physical
address of the client, which means it does not need the services of ARP for
logical to physical address mapping. However, some systems do not allow the
bypassing of ARP, resulting in the use of the broadcast address.