MAC addresses
Each device joins the network through a network card (NIC). Every card has a MAC address: 48 bits, written as six pairs of hex digits, like 00:00:5e:00:53:01. The first three pairs (the OUI) name the organisation that assigned it.
Building a frame
You want to send a small packet to laptop B. Your card wraps it in an Ethernet frame. It starts with 7 bytes of preamble (10101010 again and again) so the receiver can lock on, then a 1-byte start frame delimiter (SFD).
Next come two MAC addresses of 6 bytes each: the destination (laptop B, 00:00:5e:00:53:02) and then the source (you, 00:00:5e:00:53:01). The destination goes first, so every card can decide early whether the frame is for it.
Then 2 bytes called the EtherType say what kind of data rides inside, so the receiver knows where to hand it: 0x0800 means IPv4, 0x86dd means IPv6 and 0x0806 means ARP. Ours is 0x0800.
The payload carries the data: 46 to 1500 bytes. Our packet is only 33 bytes, so 13 zero bytes pad it to 46. Last comes the FCS: a 4-byte check number (CRC-32) worked out from every byte from the destination to the payload's end.
On the shared wire
Your card sends the frame onto the wire, one bit after another. This segment is shared, like a hub: every card on it receives every frame. So the printer and laptop B both hear this one.
The printer reads the destination, 00:00:5e:00:53:02. That is not its own MAC (00:00:5e:00:53:03) and not the broadcast address, so its card quietly throws the frame away. The printer's software never even sees it.
Laptop B reads the same destination, and it matches its own MAC. Its card recomputes the CRC-32 and gets 0x0ffbb8c5, the same as the FCS, so nothing was damaged. Because the EtherType is 0x0800, the payload goes to IPv4.
Special cases
Now your card sends an ARP request (EtherType 0x0806) to ff:ff:ff:ff:ff:ff, the broadcast address: all 48 bits set to 1. Every card on the segment keeps a broadcast, so everyone hears the question: who has 192.168.1.20?
Another frame for laptop B is hit by electrical noise and one bit flips. Laptop B's card recomputes the CRC-32, it no longer matches the FCS, so the frame is dropped. Ethernet does not resend it; higher layers such as TCP notice and resend.
The frame, field by field
;; step 1/10 · mac-address your laptop 00:00:5e:00:53:01 IP 192.168.1.10 laptop B 00:00:5e:00:53:02 IP 192.168.1.20 printer 00:00:5e:00:53:03 IP 192.168.1.30 48 bits = 6 bytes = 6 hex pairs OUI 00:00:5e (first 3 bytes): IANA's block 00:00:5e:00:53:00–ff is reserved for examples (RFC 7042 / 9542)
This example frame is real: a 33-byte IPv4/UDP packet carrying "hello", padded to 46 bytes, with FCS 0x0ffbb8c5 computed from its bytes. A hub, or an old shared coax cable, repeats every frame to every card. Modern networks use switches, which send a frame only toward the port where its destination lives.