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I have power distribution unit with an ethernet interface, that allegedly has been configured to have an IP address of (the network it was on was not on the public internet). That old network is no more, but I have a switch and a laptop running windows 7. I am trying to reach the configuration page of this PDU so that I can change some settings (like its IP address). So far I have been vexed in my attempts to do so, and I blame my rusty skills with IP networking.

For starters, I disconnected my laptop from the internet, and plug my PDU and laptop into my switch. I hardcoded the laptop's IP to, with a subnet mask of No joy, I can't ping

So I think the problem is either that the PDU is actually set to a different IP address, or I simply fail to understand how to do IPv4 networking. If it's the former, how do I figure out what the PDU thinks its IP address is? If it's the latter, what am I missing?

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migrated from Mar 11 '12 at 19:14

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What you've done should work, so perhaps the PDU does have a different IP. You could run Wireshark and see if it replies to ARP when your laptop attempts to join its network. Does it have a console port? You may be able to find or make a console cable and get into the interface that way. – goblinbox Mar 11 '12 at 19:28
Are you sure the PDU is set to answer pings? Do you know the PDU's MAC address? – Ƭᴇcʜιᴇ007 Mar 11 '12 at 19:34
What make and model PDU? – Dave M Mar 11 '12 at 20:06
I'm not sure that the PDU is set to answer pings. I know it has a web interface for configuration, but when I point my browser to I don't get a reply. – obermeister Mar 12 '12 at 1:40
I tried firing up wireshark - the only messages I see are IPv6.. It does have a serial port but I don't have the right kind of cable on hand – obermeister Mar 12 '12 at 1:41

When I'm in that situation, I usually plug my laptop into the device "back-to-back" (no switch in between; my laptop has an auto MDI-X (auto-crossover) Ethernet port, or you can use a crossover cable). Then I fire up my favorite sniffer in promiscuous mode and listen for any packets that aren't from my host:

sudo tcpdump -i $INTERFACE -nevvv not ether src $MyMACAddress

...where $INTERFACE is the interface identifier for that Ethernet port, typically en0 on my system, and $MyMACAddress is the MAC address of that Ethernet port.

Then I watch for any traffic, and see what IP address and MAC address it comes from. If I don't see any traffic within several seconds, I try to prompt it by unplugging and replugging the Ethernet cable (most devices send some traffic every time they get a link-up event), or by power-cycling the device I'm dealing with (most devices send some traffic on boot).

If all you see are DHCP requests from, then you know the device is configured for DHCP and doesn't even know how to do IPv4 link-local addresses (i.e. 169.254.x.x/16, self assigned IP addresses, or "APIPA" in Microsoft-ese). Expand your little isolated network to include a DHCP server, then see what IP address lease your device gets via DHCP (possibly by asking your DHCP server which IP addresses it gave out to which MAC addresses).

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I would say your knowledge of IPv4 is good, and you made a good attempt. However, if you truly do not know it's IP, then ARP and DHCP will not help. You'll need something advanced like RARP (find a IP given the MAC, opposite of ARP).

If your switch is also a router (e.g. wireless router), it may have its own Web interface and IP tools you can use instead of trying to get tcpdump to work on Win 7 (winpcap, tcpdump, wireshark, cygwin, etc. are functional on Win7 but un-fun for the rookie).

Here a wild stab: * plug PDU into WAN port

  • plug PC into switch port (LAN side) and let the PC use DHCP

  • try to ping the, even if it fails, it may force an ARP on the WAN side (??)

  • open router Web-UI something like (see your router docs)

  • try using the router web tools to analyze that WAN network, or at least see if you can find its MAC address?

But this may not work either, since you've effectively tried this with your direct switch attempt.

Also, many devices will never respond to PING, so the failure of PING is not surprising.

Do you know what port it's supposed to be using? Hook up both devices to the switch like you did before (with the IP and matching subnet mask) and try this:

telnet 80 (80 is the standard HTTP port, also you may need to install Telnet Client from Windows Ctrl Panel)

and if you get a blank screen, you are connected successfully! (Hit Enter a few tiems to get the prompt back)

You may want to check your ARP table. Try this from a command line : C:\Windows\system32>arp -a

if you get: No ARP Entries Found.

... then the ARP has failed, and your device likely does NOT use that IP address. Unfortunately ARP does not work unless you know the IP.

But if you get :

Internet Address Physical Address Type 48-55-35-45-55-4e static

Then you have successfully discovered the device.

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turns out the IP Address that I was given was incorrect, luckily someone else knew the correct IP Address and my problem went away. – obermeister Mar 13 '12 at 2:37
In any case, I did know the MAC address for the PDU (by looking at the bottom). I tried to look for any ethernet traffic using wireshark from that device, but it didn't appear to send any of its own, and I didn't know how to casue it to send something into the network – obermeister Mar 13 '12 at 2:39

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