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TL;DR:
I have > 2 computers in an un-managed switch which can't communicate with one another i.e. can't ping or ssh among one another.

Network Diagram

Long story: As per the diagram my router can reach the Internet and there is a computer connected to it. I have a switch (please ignore 'Layer 2' text in the diagram) connected to that router. This switch has 2 or more computers connected to it. All of the computers are hard wired. So far this is what I have been able to verify.

  1. Computer A can reach the Internet
  2. Computer A can ping and ssh Computer B & C
  3. Computer B & C can reach the Internet
  4. Computer B & C can ping and ssh Computer A
  5. Computer B cannot ping or ssh Computer C and vice versa and I get the following when pinging
From 192.168.2.141 icmp_seq=1 Destination Host Unreachable  
From 192.168.2.141 icmp_seq=2 Destination Host Unreachable  
From 192.168.2.141 icmp_seq=3 Destination Host Unreachable  

There are no firewalls setup for ICMP or SSH in Computer B and C as demonstrated by Step 1.
I'm kind of stumped as to why this is happening. Given that Switch is connected to my Router I'd figure that computers in the switch would be able to communicate with each other or maybe I'm completely off base here and missing some fundamental concept here.

Any help is appreciated. TIA!

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  • Sounds like port-isolation on the switch. That would be rare for an unmanaged switch. – Ricky Jan 17 at 10:55
  • Unfortunately, there is no configuration for an unmanaged switch , so we cannot help with that, and hosts/server configurations are off-topic here. You could try to ask this question on Super User. – Ron Maupin Jan 17 at 14:29
  • Fair enough. I was unsure if this was the right place to ask before posting this but I'll take it to SuperUser instead. Thanks. – shriek Jan 17 at 16:23
  • Are you sure it's unmanaged? – user253751 Feb 15 at 15:24
  • I should have done a follow-up on this one but when upon closer inspection on this switch there was a key in that switch that enables vlan with isolation (called one-key vlan). Once I turned it off it's able to ping again. – shriek Feb 25 at 4:50
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If both B & C share the same broadcast domain on an unmanaged switch and the same IP subnet, they should be able to see each other and there must be something actively preventing them to communicate.

The ping output indicates that they're even unable to ARP resolve each other's IP addresses, adding to the mystery.

I see a few potential causes:

  1. The "unmanaged switch" has a designated uplink port, pointing at the router. It doesn't allow the other ports to communicate with each other (very unlikely). Refer to the switch documentation, swap switch or ports to rule out.
  2. There is a MAC address conflict between B and C. Each for itself can access the Internet, but not both simultaneously (also unlikely). Check MAC addresses and simultaneous use to rule out.
  3. There is some kind of severe filtering set up on the hosts (iptables, arptables), or ARP is disabled altogether and set to static (also somewhat unlikely). Check local configs, ARP tables, and try static ARP to rule out. Note that host configurations and issues are explicitly off-topic here.
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  • Even though this post was closed I just wanted to follow up on this saying that it was that particular switch that had that issue. I tried it on different switch and it seems to be working just fine. Thank you for giving me some clue on what path to take next. – shriek Jan 17 at 17:56

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