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Subnet mask keeps changing on me!

Last response: in Networking
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September 11, 2012 8:15:32 PM

I work at a university and something strange is happening: sometimes, people in my department lose internet connectivity. When I check the adapter properties (IPV4), I notice that the subnet mask has changed itself!

We use the format 255.255.255.0 regularly and the change is always to 255.255.0.0. This appears to happen after Windows updates much of the time, but occasionally happens after a user turns their computer on.

Most of these users are using Windows 7 Pro.

The format of our IPV4 is like this:

IP: 123.456.78.910
Subnet Mask: 255.255.255.0
Default Gateway: 123.456.78.9

DNS Server: 123.456.456.123

Obviously the IP at the end is from 1-255 but just to avoid confusion I used those numbers.

Any help would be appreciated as our IT guys have no idea why this is happening!

Thanks!

More about : subnet mask changing

September 11, 2012 9:51:59 PM

Check your DHCP server.

Also, make sure you only have ONE server responding to DHCP requests. . .
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September 12, 2012 1:49:32 PM

Hi PhilFrisbie,

Regrettably, I don't have access to any of the servers...they are controlled by our IT department and they won't let us anywhere near them (which is a good thing as I don't know enough to be using them!).

I'm guessing I won't be able to solve the problem and will have to just correct the subnet mask every time it changes on us...oh well, could be worse!

Thanks for your help!
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September 12, 2012 1:56:06 PM

UrbanG said:
....I don't have access to any of the servers...they are controlled by our IT department....


This says it all right here. You should be working with your IT department, not a public forum like Tom's. I'm sure your organization has a proper way of dealing with these issues.
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September 12, 2012 2:30:07 PM

I agree, but as I mentioned in my first post, my IT guys don't seem to know what is going wrong!

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September 12, 2012 2:51:32 PM

Since no one here knows your network infrastructure all you'll get here are guesses. Sounds like it's time for an IT purge. My bet is on a borked setting on the DHCP server for your subnet.
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September 12, 2012 3:16:13 PM

Absolutely...well, with the information you have given me at least I have a better idea of what may be happening.

Thanks for your help. If I ever figure it out I'll post it here (if possible).
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September 13, 2012 3:43:15 PM

You said you "correct" the subnet mask, are you not using DHCP? If you are, you can't change the subnet mask unless you set it to manual configuration. Does it change back to DHCP when you reboot?
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September 20, 2012 2:27:23 PM

We are using static IPs...when the subnet mask changes we have to go into the IPV4 properties menu and change the subnet mask back to 255.255.255.0.

Interestingly, another issue has been noticed: we are also losing the DNS server entry on the IPV4 properties menu! A few people had lost their Internet connection and I thought it would be the subnet mask issue...when checking the IPV4 settings I noticed that the DNS server had been erased! This happened on several different machines.

As previous commenters have mentioned, likely there is a setting on a server that needs attention. Regrettably, I do not have access to the servers and I wouldn't even know what to do if I could access them! I have a feeling we may just have to live with this problem for now!

Thank you all for your help!
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September 24, 2012 2:11:16 PM

If you are not using DHCP, nothing on your server should be changing the settings. Is this happening to the same computers all the time?
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September 24, 2012 5:42:39 PM

hang-the-9 said:
If you are not using DHCP, nothing on your server should be changing the settings. Is this happening to the same computers all the time?


Yes, for the most part it seems to be the same computers with the issues.

The person who recently had their DNS automatically erased did not suffer from the 'changing subnet mask' issue...I'm wondering if the affected people are using some type of software that is causing trouble with their network settings.

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September 24, 2012 5:57:57 PM

They may be trying to get around network policies, maybe trying to change their IP manually or by using proxy servers, VPN sites, things like that.

If the users are local admins, remove admin rights for a bit, see if the issue goes away.
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October 1, 2012 8:55:48 PM

Interesting...I hadn't thought of that!

Thanks for the advice!
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