While trying to setup a MSSQL clustering solution, I am running into a problem that is outside of my expertise that is related to networking.

I was trying to find a free IP to used for my node. I asked the network admin to give me a free IP address. And for argument sake he gave me an IP of which supposed to be unused.

When trying to use the given IP during clustering setup, SQL complaint that the IP has been used.

I tried pinging the IP from my server and I got a reply back from say...

Any idea what might be causing something like this and more importantly how to troubleshoot it?

  • btw.. what IS Is it a computer or a network device? {router, switch, etc..}
    – mpeterson
    Dec 22, 2009 at 22:38

6 Answers 6


This isn't totally unheard of, a machine may have had an alias assigned to a network card previously which the network admin doesn't know about, etc.

The obvious thing to do is find the MAC address of the responding server, that should be easy to do.

Run "arp -a" from a command prompt on the SQL server, and look for both the and addresses.

You'll see a MAC address for both, which might or might not be the same, they'll look like "00-22-6b-3b-30-90". Your network admin should be able to identify the network card with the MAC addresses listed, but it's probably worth checking that it's not one of the SQL server network cards before you tell him, just in case it's not a minor configuration mistake you've made yourself somewhere along the line.

To check a windows system MAC address run "ipconfig /all" from the command prompt, and you'll see a MAC address for each network card.

  • +1 - the Network Admin should be figuring this out, and the MAC is definitely the place to start.
    – mpeterson
    Dec 22, 2009 at 22:34
  • Currently, as arp is being deprecated, one should use ip n instead.
    – Akito
    May 4, 2020 at 14:26

Several possibilities:

  1. Broadcast address. If your "let's say" examples weren't precise, you could have been pinging the broadcast address on the subnet. Let's say the network was (AKA mask of and you pinged - it may look like just a random host, but it's really the broadcast address, and anyone on that subnet could respond with their own address (FYI the broadcast address is the last address of the subnet).

  2. VIP. If you have load-balancers or a server cluster, you may have pinged the shared/virtual address and gotten a reply from the real server.

  3. Secondary addresses. If the router/server has a primary address of .69 with a secondary of .205 (common practice for people trying to reserve extra addresses seen by ping scans just like yours) it might reply to a ping for .205 but source the reply from .69.

  4. Your own address. An example from a Unix laptop: ping responded with From icmp_seq=1 Destination Host Unreachable where was the laptop's IP address.


I'd like to see the output. Are you sure it wasn't a ICMP error message?

If your subnet mask were configured incorrectly, the router might respond to the ping. What was the subnet mask you were given?


If you have a network admin assigning IP addresses, the same network admin should be able to troubleshoot the response. It's possible that a machine has multiple NICs (and therefore multiple IP addresses).

  • Hmm... not quite the answer that I am looking for. The network admin is "sure" that the particular IP address he assigned to me is free. So let's just say that he might not know where the problem as well... Dec 17, 2009 at 4:27
  • 1
    But wouldn't he be able to ping it and see that it is not free? Unless he is incompetent or lazy, I'm sure he would want to know about this and resolve it (or at least give you a new IP).
    – Beep beep
    Dec 17, 2009 at 4:44
  • Hmm... let me put it a different way... Is it possible that when he ping it it does not get any response and when I ping it, it is replying through a different IP address? If so, what might be causing it? Dec 17, 2009 at 5:15
  • If you're on different subnets or separated by any kind of firewall/router then it's possible that you could see different results if there's a screwy configuration in there.
    – Chris W
    Dec 17, 2009 at 10:10

Try to resolve that IP address into a hostname. ping -a <IP> in Windows. Linux can use host <IP> or dig -x <IP>. The hostname of that IP address might be useful to you or your network admin. (I'd try resolving both IPs you're dealing with)

It's also not impossible that the ping response might be an ICMP error message -- you can try dumping the actual packets with tcpdump/Wireshark, and see exactly what you're getting in response.

You can also try connecting to that IP address with HTTP, HTTPS, SSH, telnet, RDP, etc. If any of those connect, you'll probably get some real useful info on what the machine is.


Given where it appears to reside on your network it sounds like a VIP to me, does it always respond back from the same IP? Is it's MAC address come from a valid NIC-vendor's range?

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