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The maximum size is 1500 Bytes. Is there a lower limit? 1? 64? Must it be a multiple of two? Is there somewhere I can look to learn more?

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Yes, IPv4 implementations require a minimum MTU of 68 bytes per RFC 791.

See Peter's answer for more detail.

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    You contradict yourself. The quote says "up to 576", not "at least 576". – Nobody Jun 5 '15 at 21:22
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    Edited answer for clarity. Devices can receive packets with smaller MTU's, but per the RFC, 576 bytes was the right size to use. Most OSs enforce a number around 576, it varies slightly - so there's really no reason to send smaller than necessary. – Jordan Head Jun 7 '15 at 13:22
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    This answer is wrong, it's confusing datagram size with MTU. – Peter Green Mar 7 '16 at 13:11
  • Yup, you're right - edited my answer. Thanks for catching that @PeterGreen. – Jordan Head Mar 7 '16 at 21:18
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The minimum MTU for IPv4 is 68 bytes. Specifically from https://tools.ietf.org/html/rfc791

Every internet module must be able to forward a datagram of 68 octets without further fragmentation. This is because an internet header may be up to 60 octets, and the minimum fragment is 8 octets.

However such a low MTU would be extremely inefficient.

IPv6 sets a much higher minimum of 1280 bytes and requires links that can't support that MTU to provide a link-specific fragmentation and reassembly layer. From https://tools.ietf.org/html/rfc2460

IPv6 requires that every link in the internet have an MTU of 1280 octets or greater. On any link that cannot convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6.

Presumablly the motivation is to stop low MTU links from driving inefficiency across the network. I assume 1280 bytes was chosen to allow for a packet to be wrapped in some encapsulation/tunneling crap and still fit in a standard ethernet frame.

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  • So are there some reasons in a real-world IPv4 networks to set MTU to 68 or e.g. 200 or something very small relative to the usual 1500? – red0ct Sep 22 '20 at 15:15
  • IPv4 was designed in an era when connections were much slower. If you are running over a 1200 baud modem then a 1500 byte packet will block the link for over a second, this is generally not desirable. – Peter Green Sep 22 '20 at 15:21
  • Thanks! In other words MTU value is determined by the physical transmission medium (which in turn is determined by the devices). So if there are some slow devices with corresponding link-layer protocol between them - there could be some small MTU value. Isn't it? But I'm not sure that this is the case in modern IP-networks. – red0ct Sep 22 '20 at 16:04

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