Which port does a DHCP client use for DHCPDISCOVER?

N10-009 By islever Editorial Last updated

Explain common networking ports, protocols, services, and traffic types. Given a scenario, implement IP v4 and IP v6 network services.

Short answer

B A new machine sends its DHCPDISCOVER from UDP port 68, broadcasting to the DHCP server on port 67.

Question

Which of the following ports is commonly used for DHCPDISCOVER call requests for a new machine on a network?
  1. 53
  2. 68 Correct Answer
  3. 69
  4. 80

Key Takeaways

It tests whether you can separate the client port from the server port in DHCP, with the trap being to substitute the later TFTP (69) or DNS (53) stage of a boot flow for the discovery itself.

This question tests the DHCP port pair, specifically the client-side UDP port used for the initial DHCPDISCOVER broadcast. The correct answer is B (68), because the client sends from 68 while the server listens on 67.

Common mistake: Picking 69 (C) because DHCP often precedes a TFTP/PXE boot download, but TFTP is only contacted after the client has received a lease and a boot filename, not during DHCPDISCOVER.

Expert Analysis

The client side of DHCP is port 68 A host that has just been powered on owns no IP address, so it cannot address a server directly. The DHCP client process binds UDP port 68, then broadcasts a DHCPDISCOVER with source 0.0.0.0:68 and destination 255.255.255.255:67. The question asks which port is used for the DHCPDISCOVER call from the new machine, so the client port — option B, 68 — is the answer. Port 67 belongs to the DHCP server/relay side and is deliberately absent from the option list, which is what makes this question feel harder than it is.

Why the other options look plausible Option C (69) is the strongest distractor in practice: many enterprise boot flows chain DHCP into a PXE/TFTP download, so candidates remember "DHCP plus 69" as one story. But TFTP only starts after the client has a lease and a boot filename from DHCP options; the DISCOVER itself never touches 69. Option A (53) is attractive because a fresh machine genuinely needs name resolution, and DHCP option 6 hands out DNS server addresses — yet resolution happens after addressing, not during discovery. Option D (80) appears in captive-portal and proxy auto-config scenarios, which again occur only once the host has an address.

What the comments confirm Respondents describing the protocol consistently split the pair the same way: port 67 is used by the DHCP server, while port 68 is used by the client "to send and receive DHCP messages, including the initial DHCPDISCOVER request message," and clients "broadcast their initial DHCP Discover messages to the local network segment using port 68." That framing matches RFC 2131 and settles the client-side question.

The single point where the trap breaks The distractor set stops being correct the moment you ask who is speaking. The DISCOVER is client-originated, so it is port 68; reserving 67 for the server and 69 for the later TFTP boot stage keeps the whole exchange straight.

Official Reference

https://www.rfc-editor.org/rfc/rfc2131

This link opens the official vendor documentation in a new tab.

Exam Strategy

When a question names a specific message such as DHCPDISCOVER, DHCPREQUEST or DHCPACK, ask yourself which side originates it before looking at the ports. Client-originated DHCP traffic uses 68; server and relay traffic uses 67. Then scan the options for the port that belongs to a neighbouring service in the same workflow — TFTP 69, DNS 53, HTTP 80 — and eliminate it, because a later stage in the boot chain is never the port the named message uses.

Frequently Asked Questions

Does a DHCP client send DHCPDISCOVER from port 68 to port 67?

Yes. The client broadcasts from UDP source port 68 to destination port 67, where the DHCP server or relay is listening, because the host has no IP address yet.

Which port does the DHCP server use to send its reply?

The server or relay sends from UDP port 67 back to the client on port 68, either broadcast or unicast depending on the flags and relay configuration.

Is port 69 involved in DHCP?

No. Port 69 is TFTP, which may be used later in a PXE boot to download a boot file after DHCP has supplied an address and boot options.

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Discussion 2 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 2

  1. ERB14 👍 1 Answered B
    Port 68 (and Port 67): DHCP (Dynamic Host Configuration Protocol) uses UDP ports 67 and 68. Port 67 is used by the DHCP server, while Port 68 is used by the client to send and receive DHCP messages, including the initial DHCPDISCOVER request message.
  2. 1c8618a 👍 1 Answered B
    DHCP clients broadcast their initial DHCP Discover messages to the local network segment using port 68.

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