Recap: Physical and Data Link Layers

A signal on a medium, a frame to the next device, and an address that changes at every hop.

The bottom two layers carry data across one link, from a device to the next one.

The physical layer turns bits into voltage, light or radio. The medium sets speed and reach, and nothing here reads an address.
A frame carries one packet across one link, addressed by MAC address, with a checksum at the end to detect damage.
A MAC address names an interface on a link; an IP address names a machine across networks.
A switch learns which port each source MAC is on, forwards known destinations to one port, and floods unknown ones and broadcasts.
ARP broadcasts "who has this IP address?" and the owner replies directly. For a remote destination, a host asks for its router's MAC.
The MTU caps the size of a packet on a link. A packet too large is fragmented or dropped, and blocked ICMP turns that into a silent hang.
MAC addresses change at every hop; IP addresses do not, except through NAT.

The bottom two layers, what each holds, and what it is for
Layer	Holds	Lives for
2 Data Link	Frames addressed by MAC; switches and ARP	Reaching the next device on the link
1 Physical	Bits as signals on copper, fibre or radio	Carrying bits as fast and as far as the medium allows

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