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drodecker
Asked: 2015-02-13 10:16:48 +0800 CST

Why does a router ping failure respond using a different IP?

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From Windows CMD, when trying to ping another computer on our internal LAN, the IP obviously fails to respond. However the reply of "Destination host unreachable." comes from yet a different IP, and not that of the router.

In my network, the gateway is 10.0.0.252, but I'm getting a reply from 10.0.20.188; which is an dynamic DHCP allocated IP to general devices.

C:\Users\drodecker>ping pbx

Pinging pbx.office.relevantads.com [10.1.1.211] with 32 bytes of data:
Reply from 10.0.20.188: Destination host unreachable.
Reply from 10.0.20.188: Destination host unreachable.
Reply from 10.0.20.188: Destination host unreachable.
Reply from 10.0.20.188: Destination host unreachable.

Ping statistics for 10.1.1.211:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
windows

1 Answers

  1. SIMPLE ANSWER: Your PC is trying to resolve the Ethernet MAC address of the next-hop interface, but it can not find none (eg: the target PC is powered off). So it reports a "Destination host unreachable" ICMP message originating from its own interface.

    LONG ANSWER An IP-over-Ethernet network has two complementary network addresses: a non-routable, link-layer physical addressing (the MAC address) and a routable, network-layer logical addressing (the IP address).

    When trying to ping another hosts, your PC must:

    1. decide, by virtue of the routing table, the outgoing interface
    2. determine the MAC address of the next-hop Ethernet interface (using the ARP protocol)
    3. send a packed with the next-hop interface as the destination MAC address and with the target IP address as the final destination

    Point n.2 is were behavior can diverge:

    • if the remote host is on the local network, the next-hop Ethernet interface is the final destination you are trying to reach (eg: if my PC has IP 10.0.0.1 /24 and I am trying to ping PC with IP address 10.0.0.2, the next-hop interface is that of the target computer);
    • if the remote host is on a remote network, the next-hop Ethernet interface is that of the gateway interface (eg: if my PC has IP 10.0.0.1 /24 and I am trying to ping PC with IP address 10.10.10.1 /24 and that remote network can be reached using local router 10.0.0.254 /24, the next-hop interface is that of the router).

    So, if your ping failed with a reply from your own PC interface, we have two possibility:

    1. you are trying to ping a local address: this address is not replying to your PC ARP requests and, after a timeout, your PC inform you that the target Ethernet address can not be found.
    2. you are trying to ping a remote address: your configured gateway for that address (eg: your default gateway) is not responding to your PC ARP requests. After a timeout, your PC inform you that the target Ethernet address can not be found.
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