Inside the cable
Your laptop plugs into a wall socket, and a cable inside the wall runs on to a switch. It is copper Ethernet cable, and bits travel along it as changing voltages.
Cut the cable open and you find 8 thin copper wires, twisted together in 4 pairs. Each pair has its own colour, orange, green, blue or brown, plus a white-striped partner.
Electrical noise from motors or other cables hits both wires of a pair almost equally. The receiver reads only the difference between the two wires, so the noise cancels out.
Plugs and limits
Each end has a clear plastic plug with 8 metal pins, one per wire, and a clip that locks it in. Its proper name is 8P8C (8 positions, 8 contacts), but everyone says RJ45.
The T568A and T568B standards say which colour goes on which pin. They only swap the orange and green pairs. Use the same order at both ends and every pin meets its partner.
Cables come in categories, printed on the jacket. Cat5e carries 1 Gb/s for 100 m. Cat6 carries 10 Gb/s, but only for about 55 m. Cat6a carries 10 Gb/s for the full 100 m.
One Ethernet run may be at most 100 m: up to 90 m inside the wall plus 10 m of patch cords. Any longer and the signal gets too weak and noisy. The trip itself takes only about half a microsecond.
The same cable can carry power. With PoE (Power over Ethernet) the switch sends about 50 volts along the pairs, so a Wi-Fi access point on the wall needs no power plug. The newest standard allows up to 90 W.
Other cables
Coaxial (coax) cable is round and built in layers: a copper centre wire, a plastic insulator, a metal shield and an outer jacket. The shield wraps all the way round, keeping noise out and the signal in.
Coax screws onto a wall outlet with an F-connector; the centre wire itself is the pin. It brings cable TV and cable internet into a home, where a cable modem turns the signal into Ethernet.
Your laptop has other plugs too. USB-C (24 pins, fits either way up) carries data, power and video to nearby gadgets. HDMI (19 pins) sends picture and sound to a screen. Both reach a few metres, not across a building.
The details
;; step 1/11 · wired-link laptop ── patch cord ── wall socket ── cable in the wall ── switch medium copper twisted pair (Ethernet, IEEE 802.3) signal voltage changes between the two wires of each pair
The plug, the cut-open pairs and the coax are drawn magnified; beads stand for the signal and are slowed down billions of times. Categories follow ANSI/TIA-568.2-D. 10GBASE-T (IEEE 802.3an) reaches about 55 m on Cat6 (TIA TSB-155) and 100 m on Cat6a.