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I'm just starting my IT education and this is probably a very basic question but I'm having trouble with it.
As far as I understand a collision domain is a part of a network where collisions can occur.
Let's say we have two PCs connected to a switch and nothing else.
Why is each port considered a collision domain when there's no collisions that can occur?

2 Answers 2

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A collision domain is created by a port running in half-duplex mode (due to requiring media arbitration using CSMA/CD). The collison domain spans all ports within the same layer-1 segment - it crosses repeaters (hubs) but ends at host, switch or router ports.

Half-duplex and CSMA/CD are all but obsolete. Practically, they only existed for 10 and 100 Mbit/s Ethernet. Gigabit and faster speeds use full-duplex transmission throughout. With full duplex there's no media arbitration and therefore no collision domain anywhere.

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  • I see, so in my example there actually wouldn't be any collision domain, if all ports run full-duplex?
    – Karan Vess
    Mar 1 at 12:23
  • Yes, exactly. If each host and the switch support full-duplex mode (a given for all devices from this century), that mode is auto negotiated and used. Even if half-duplex mode was used, each switch port creates a collision domain of its own. Of course, each switch port can run in half or in full duplex, independent from each other.
    – Zac67
    Mar 1 at 13:38
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Suppose you connect on this port a hub (yes they are increasingly rare but still exists)

Now you have a real collision domain on your port.

You are right that in practice this is less and less a concern, but still the principle remains.

Also the fact is most network courses are vastly outdated (if your course talk about classful network - class A, B, C etc.. it is totally outdated)

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  • So basically it's only a collision domain because a half duplex device COULD be connected to it? Apparently this is part of the CCNA cert, so ¯\_(ツ)_/¯
    – Karan Vess
    Mar 1 at 12:20
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    @KaranVess That answer isn't quite correct as a collision domain could very well span just two ports, the switch port and the host port, when both are operating in half-duplex mode.
    – Zac67
    Mar 1 at 13:49

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