DHCP messages from a client to a server are sent to the 'DHCP server' port (67), and DHCP messages from a server to a client are sent to the 'DHCP client' port (68).
If the S is used as a Relay agent:
- Client sends DHCP discover and Request messages with Source port 68 and Dest port 67
- Relay agent forwards using Source port 68 / Destination port 67 to the DHCP server
- Relay agent converts server response back to Src67 / Dest68 to the client.