I have two computers: PC1 and PC2. PC1 has an IP address of, and PC2 has an IP address of They are connected directly to each other with a crossover cable. My understanding is that they should not be able to ping each other, but I just tested it, and they can both ping each other. Have I misunderstood something about how subnets work?


EDIT: I can change the IP addresses and subnet masks to anything I want, and they can still ping.

  • Maybe there's some kind of proxy ARP thing they are running. Aug 5, 2015 at 13:09
  • What do you have set as your default gateway? If its the other computer's IP address, then that would be why it is working.
    – Eddie
    Aug 5, 2015 at 13:32
  • I have not configured a default gateway on either computer.
    – albinotuba
    Aug 5, 2015 at 13:34
  • What OSs? What does tcpdump/wireshark on the receiver say about who is sending the ICMP traffic? Are you sure that you're using /30 prefix, and that only one IP/prefix is configured on the NICs involved? Are there any other configured NICs on the system (connected to a mutual network)?
    – stevieb
    Aug 5, 2015 at 14:14
  • Mac OS 10.10 on one and 10.8 on the other. Wireshark shows an ARP broadcast from the pinging computer ( asking who has, and then responds with its MAC address, and then they ping. I don't see anything out of the ordinary going on in Wireshark. I'm almost completely certain I configured the subnets correctly. It's always possible that I made a stupid mistake. I'm using on both computers. They both have Wifi and other NICs, but they are all disabled. Only ethernet is active.
    – albinotuba
    Aug 5, 2015 at 14:27

1 Answer 1


I have seen this before on other host types. Putting in default gateway addresses should prevent this behavior. Without a default gateway, the host sends an ARP for the other host's layer-3 address, and it receives a reply because the hosts are on the same layer-2 domain. With a default gateway, the host would ARP for the gateway's layer-3 address, not the other host's layer-3 address, but there is no gateway address for which to ARP.

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