What a topology is
Six devices need links to talk. A topology is the shape of those links: who is wired to whom. The physical topology is how the cables run; the logical topology is how the data actually flows.
Bus
In a bus, every device taps into one long cable, like old thin-coax Ethernet. A's message spreads along the whole cable and every device hears it; only B keeps it. If two devices send at once, their signals collide.
Cut the bus anywhere and the whole network fails, not just one half. The cut end has no terminator, so signals bounce back and garble everything. One cable means one weak spot shared by every device.
Star
In a star, every device has its own cable to a central box, a switch. A sends to the switch, which passes it only to B. Today's wired Ethernet works this way; Wi-Fi does the same around an access point.
Cut one device's cable and only that device drops off; A still reaches B. Adding a device is just one more cable. The weak spot is the centre: if the switch fails, everyone is cut off.
Ring
In a ring, each device links to exactly two neighbours, and data travels round in one direction. C passes A's message on to B. Old Token Ring offices worked like this; today fibre rings join sites across a city.
One cut stops a single ring, because data only goes one way. So city rings add a second ring running the other way. Traffic goes round the long way and still reaches B, switching over in under 50 milliseconds.
Mesh
In a full mesh, every device has its own link to every other. Six devices need 6 × 5 ÷ 2 = 15 links; twenty would need 190. That is a lot of cable, but A always has a direct path to B.
Most meshes are partial: only some pairs are linked. Cut the A–B link and the message simply goes round through C. The internet's core works this way, and so do Wi-Fi mesh and Zigbee smart-home networks.
Tree, hybrid, logical
A tree joins small stars together: devices plug into nearby switches, and those switches plug into a central one. Offices and campuses are built like this. It grows easily, and each cable stays short.
Cut the cable above a branch and every device below it is cut off from the rest, here B and D. A still reaches C. Real networks are hybrids that mix shapes: stars at the edge, a mesh in the core.
Swap the switch for an old hub. The cables still form a star, but a hub repeats every message out of every port, so every device hears it, just like a bus. Physically a star; logically a bus.
What is happening
;; step 1/12 · shape ;; about: a, b, nodes devices A · B · C · D · E · F links none yet topology the shape the links make physical where the cables go logical the path the data takes
10BASE2 (IEEE 802.3 clause 10): 50-ohm thin coax, segments up to 185 m, at most 30 stations, a terminator at each end. Stations share the cable using CSMA/CD (IEEE 802.3 clause 4): listen first, and back off after a collision. Token Ring is IEEE 802.5 (4 or 16 Mbit/s). FDDI (ISO 9314) used two counter-rotating fibre rings; metro Ethernet rings today use ITU-T G.8032 protection, which aims to switch over in under 50 ms. At 2/3 the speed of light, fibre adds about 5.0 µs per km, so the long way round this 10 km-per-hop ring adds about 100 µs.