Unexpected IPv4 address when pinging machine on home network

Question

Set-up

I have three computers on a home network. No domain, just using Windows "workgroup" functionality. I'm using Input Director from a desktop Win7 PC to control two laptops (one Win7, one WinXP)

Problem Symptoms

Everything was fine until I rebooted the Master. When it came back up Input Director was working with "barney" but not with "laptray". I've found that the problem is a more general networking thing where Input Director was simply the first thing I noticed fail. Subsequently this is what happened:

• Pinging laptray from master gave "hostname not found".
• ipconfig on laptray returned expected IP address for my local network.
• Pinging laptray from barney (which hadn't been rebooted) succeeded but gave an unexpected hostname and IP address of laptray.cable.virginmedia.net [81.200.x.x].
• While I was busy debugging this issue and starting to write-up this question, I noticed that Input Director picked up laptray by itself and started working correctly.
• Now that it's working again, I see that pinging laptray from Master reports the correct local network IP address as reported by ipconfig on laptray.

The Questions

1. How can it be that ipconfig on laptray reports a different IP address than pinging it from barney?
2. Where did laptray.cable.virginmedia.net [81.200.x.x] come from and how do I rid myself of it?
3. Why did it take some minutes (5-10) after the reboot of master for it to start working?

UPDATE 1

It seems that laptray.cable.virginmedia.net isn't my laptop - it's another machine that has a similar name. The problem now becomes

Why does laptray resolve to laptray.cable.virginmedia.net [81.200.x.x] when pinged from barney, but resolves correctly when pinged from Master?

Answer 1

For those that are interested, this explains what is happening....so that it's not so scarey when you ping a machine on your network/or tracert it, and it goes to an external IP !

http://www.cableforum.co.uk/board/showthread.php?t=33666299

It's due to VirginMedia's DNS service...which when you specify an unknown host....tries to redirect you - this service can be turned off.

81.200.64.50 is what I got.

Answer 2

How can it be that ipconfig on laptray reports a different IP address than pinging it from barney?

Perhaps because you have multiple inconsistent sources of name-to-address mapping in your system. For example you might have hosts files on some PCs that are incorrect.

Where did laptray.cable.virginmedia.net [81.200.x.x] come from and how do I rid myself of it?

Perhaps your router told barney that the router knows about DNS names and addresses and that it was part of the cable.virginmedia.net domain. barney asked the router about "laptray", the router said no such domain so barney appended the domain and retried.

Why did it take some minutes (5-10) after the reboot of master for it to start working?

Perhaps because, in the absence of a Windows domain controller or a local DNS service, Windows falls back to listening to occasional broadcasts (WINS) from other PCs and it takes a while to build up a picture of what's around that way.