Let's say for example we have the following hypothetical topology:
In each network, can each user communicate with each other?
For example, can User 1 ping User 2?
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It always depends on how the operator configured it. Note that if User 1 can't ping User 2, their internet is broken - no buts about it, that's a fault. But I notice this is a private IP address range (RFC1918) which means this carrier is probably using CGNAT. Which makes it more ambiguous and dependent on their specific configuration, but they've probably made it so User 1 can't ping User 2 for security reasons. – user253751 Dec 15 '20 at 20:29
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Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer. – Ron Maupin♦ Dec 31 '20 at 5:17
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can each user communicate with each other?
Ultimately, yes, as that's the point of any internet connection. The question is thus where do these two rivers meet. Routing (layer-3) or switching (layer-2) can happen at the DSLAM/OLT, or it can happen further into the network, or even in a 3rd party's network. (In resold situations, users could be isolated into different logic networks. I.e. with standard ethernet switching, if I'm on port 1 in vlan 10, and you're on port 2 in vlan 20, we cannot talk to each other directly, we may actually be many layer-3 hops apart.)
If the ISP is not providing end users with public addresses [CGN - Carrier Grade NAT], then who can see who depends on how the specific network is setup.
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Actually I mean can they communicate without using router. Forget the real world setup. If this is a lab setup, is communication possible? That's what I'm asking. – Noob_Guy Dec 16 '20 at 11:20
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Do DSLAM and OLT function like a switch?
On the physical (L1) and data link (L2) layers ("like a switch"), each CPE can only communicate with the DSLAM or OLT (COE). If those were to forward data between CPEs they could communicate directly, but that is highly unlikely. Forwarding L2 between CPEs would open the door to various attacks including MAC/IP spoofing, MAC or ARP flooding, so it's usually filtered.
Some AMs route already, so L3 communication (see below) might even work directly.
can User 1 ping User 2?
Of course, the users can usually communicate across the access server (BRAS) off the diagram to the far right. So ping, working on the network layer (L3), should be working between users.
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But I would imagine that with most regular residential or business ISPs, customers can still communicate with each other, even if has to take a more indirect path than what OP describes. (It would be a bit odd if, as a home internet user, I couldn't send packets to the same ISP's customers -- or if I could talk to hosts in another city but not to those in the same street...) – user1686 Dec 15 '20 at 16:31
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Sure - all the way to the access server and back again. The way the question was asked I don't think the OP was referring to that. – Zac67 Dec 15 '20 at 17:52
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@user1686, actually, most ISPs will prevent residential customers from directly communicating, sometimes totally, and sometime forcing the communications throug the ISP CO. This is a protection for the residential customers from each other. – Ron Maupin♦ Dec 15 '20 at 19:33
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@RonMaupin: Yeah, I'd certainly expect some amount of filtering to occur (risky ports, address spoofing, various L2 shenanigans that I've done, etc.), but I'm much less sure about "totally". Assuming we're talking about regular TCP/IP, I fail to understand how packets from nearby customers would be more risky than the exact same stuff coming from more distant sources – the only way it makes sense for me is if those ISPs are completely blocking inbound connections from any source, not just from other local customers. – user1686 Dec 15 '20 at 20:56
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@user1686 L2 communication between CPE is completely filtered/impossible. They can only communicate via L3 across the access router. L2 communication would indeed be very risky, enabling MAC/IP spoofing, MAC or ARP flooding, ... – Zac67 Dec 15 '20 at 21:02
