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Experiencing subnet mask clashes?

By infinityinfinity2 ·
I got 500 computers on a network using dynamic ip addressing by DHCP operating at class B (255.255.0.0). Each 4 computers are connected to a switch. The network oftenly experiencing subnet mask clashes that is the subnet mask automatically changes from class B to class A(255.0.0.0). anyone ever experienced this problem? any concrete solution or reason why this abnormal situation oftenly occur on my network?
I have only one DHCP server running on windows server 2000. Despite that the 500 workstations dont have the right to change the ip address or subnet mask still the subnet mask change from class B to Class A. That is when i go to the Local Area Connection, choose Properties, choose IP Protocol, click Properties, and see if the subnet mask i noticed that it has been change from 255.255.0.0 to 255.0.0.0. or even when i check it using ipconfig. any suggestion why?

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A rogue server -- possibly

by w2ktechman In reply to Experiencing subnet mask ...

I ran into a similar issue some years ago, and after a few hours, it was traced to 1 unattended computer. Someone was trying to learn Linux, while connected to the corp network. It wouldnt have been bad except they setup a dhcp server and hosted other server functions.
Similarly it happened on another site where someone was studying for their MS certs.

For wireless, I have seen this happen several times too. Someone will bring in their own router/dhcp server, etc. and screw things up because they didnt set ti up right.

It may just be related to a server popped up, that should not have.

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