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What concrete rule or layer of the OSI model should be used to decide whether a network is a point-to-point network or a broadcast network?

A general definition (*) seems clear, but in practice it doesn't seem so simple to me. Can you validate, complete or correct the following cases:

  1. two PCs: PCA and PCB connected directly by a cable --> p2p network
  2. two Pcs: PCA and PCB (same VLAN or not) connected by a switch --> broadcast network (due to the switch)
  3. three PCs: PCA connected directly to PCB connected directly to PCC; PCB has two NICs --> p2p network
  4. three PCs and 2 switches: PCA -- Switch1 -- PCB -- Switch2 --PCC; PCA connected to PCC via VPN --> p2p network
  5. two routers: directly connected to each other --> p2p network
  6. three routers: R1 directly connected to R2 and R3, R2 directly connected to R1 and R3, and therefore R3 directly connected to R1 and R2 --> 3 p2p networks
  7. three routers linked together by a switch --> broadcast network

Thanks in advance !

(*) A p2p link can only connect two nodes. With a broadcast link (p2mp), more than two nodes can be connected

2 Answers 2

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It depends :-)

From a topology point of view, your answers are correct.

But I assume you're talking about Ethernet, and Ethernet is (was) a broadcast medium. So in that sense, all your cases are broadcast.

But how you configure your interfaces can change that. Switched Ethernet uses full-duplex links, which electrically are point to point. The switch replaces the physical "bus" medium of the original Ethernet of your grandfather.

The bottom line is the answer depends on why you're asking. If you're attempting to configure a router or switch, you can go either way, depending on your needs.

If you're having a philosophical discussion on network topologies, then your answers are essentially correct.

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  • Thanks ! "Ethernet is (was) a broadcast medium" --> Can you explain why Ethernet was a broadcast medium, but would no longer be?
    – PanCho
    Mar 5 at 15:27
  • I was trying to point out that switched Ethernet is electrically point to point between the PC and the switch. Perhaps I overstated the point.
    – Ron Trunk
    Mar 5 at 17:32
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Depending on context, a broadcast network is a multi-node network where everyone can talk to anyone else (not necessarily at the same time). The most common examples today are Ethernet and Wi-Fi. Even if you only use a P2P topology with Ethernet - a LAN cable between two NICs, the network stays a broadcast one. Whether there's a switch or not isn't relevant.

A peer-to-peer network is a two-node connection where the one node can only talk to the other. The most common example is a serial connection between two serial ports.

The difference is the need for addressing in a broadcast network and the complete uselessness of addresses in a P2P network. It's the protocol that defines the type, not the current topology - all your examples are topologies.

(I'm referring to link-layer networks above. The ubiquitous IP protocols work with broadcast and P2P links alike.)

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  • Thank you. Is it correct to say that the main point of your explanation is that : "the protocol defines the type, not the current topology" ? Subsidiary question : in point-to-point link between two hosts, can packets follow multiple routes ?
    – PanCho
    Mar 5 at 15:15
  • Basically, yes. Ethernet is always a broadcast network, a simple serial link is always a point-to-point network/connection. In a P2P network, (L3) packets can be routed between routing hosts on the network layer, with each connected host pair using a distinct network=L2 segment=L3 subnet. Each L2 frame can only follow the P2P link it was sent on though.
    – Zac67
    Mar 5 at 16:36

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