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Asked: 2026-05-17T18:55:31+00:00

A bit of background. I’m writing an application that uses UDP. The application will

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A bit of background.

I’m writing an application that uses UDP. The application will run on a LAN (not internet).
I’ve been assuming that if my MTU is 1500 then thats how big a UDP payload can be, but I’m
not sure if the UDP header is meant to fit within that too.

I’m suspecting that if I send a UDP packet with a 1500 byte payload and the machine MTU is 1500 bytes will it end up sending two packets?

Searching the internet for a clear answer here seems harder than it should be, I’ve seen conflicting information.

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1 Answer

  1. Editorial Team
    ------------------------------------------------------------------------------
|Ethernet  | IPv4         |UDP    | Data                   |Ethernet checksum|
------------------------------------------------------------------------------
  14 bytes    20 bytes     8 bytes    x bytes                4 bytes
           \ (w/o options)                               /
            \___________________________________________/
                              |
                             MTU

    If your MTU is 1500, you have 1500-20-8 = 1472 bytes for your data.

    • If you exceed that, the packets will be fragmented ,i.e. split into more packets.
    • There might be more layers involved, e.g. 4 byte a vlan header if you’re on top of a vlan ethernet.
    • Some routers inbetween you and the destination might add more layers.
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