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In most client-server-applications, the port number of a server is a well-known number, while the client uses a currently available port number. DHCP is different. Here, both the client and the server use a well-known port: UDP port 67 for the DHCP server, and UDP port 68 for the DHCP client.

Why did they design it like that and have a fixed client port number?

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up vote 5 down vote accepted

This is necessary as the client may not have an IP address when using DHCP (he uses DHCP to get an address).

The server and the client communicate via broadcasts so the server broadcasts the IP address of the client to UDP port 68. There is no other way for the server to reach the client. And this is the only way for the client to receive an answer to his request as he has no IP address.

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