The MTU (Maximum Transmission Unit) is the largest payload an Ethernet frame can carry, 1500 bytes for standard Ethernet (the frame payload field is 46-1500 bytes). It matters because the IP layer above must fit its packets within the link MTU: a larger IP packet either gets fragmented into multiple frames (with reassembly overhead and a higher loss-amplification risk, since losing one fragment drops the whole packet) or, if the "don't fragment" bit is set, is rejected with an ICMP "fragmentation needed" message that drives Path MTU Discovery. Mismatches and black-holed ICMP cause classic bugs where small packets work but large transfers stall. On the application side, knowing the MTU helps you size messages to fit in one frame and avoid fragmentation, and TCP negotiates a Maximum Segment Size (MSS) derived from the MTU to keep its segments within it. Jumbo frames (up to ~9000 bytes) exist on some networks for efficiency but require every device in the path to support them. So the MTU is a hard per-link limit that shapes IP fragmentation, TCP segment sizing, and how you frame application data.
Networking & IoT · Interview question
What is the Ethernet MTU and why does it matter?
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From the lesson
Ethernet Basics
The dominant link layer: MAC addresses and frame format, the MAC-vs-PHY split (and how MCUs pair an on-chip MAC with an external PHY over RMII), and how switches forward by MAC.