In LAN network , computers are not accepting 10.12... series IP address . Please

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On assigning any of these IP address, PC shows either IP conflict or on pinging shows following message: “PING transmit failed, error code 1231"

But on connecting with separate small LAN, same PCs behave well.
 


On assigning any of these IP address, PC shows either IP conflict or on pinging shows following message: “PING transmit failed, error code 1231.”
But on connecting with separate small LAN, same PCs behave well.
 


So you are assigning them manually? Probably another PC has the same IP, hense the IP conflict. For the transmit failed error, here is a post with some things to try http://answers.microsoft.com/en-us/windows/forum/windows_vista-networking/ping-transmit-failed-error-code-1231-windows-vista/0b3216d3-481e-43ca-b222-e55faf56cac2
 
[On assigning any of these IP address, PC shows either IP conflict or on pinging shows following message: “PING transmit failed, error code 1231"

But on connecting with separate small LAN, same PCs behave well
 


This i have checked .. nothing like that ..
any other idea?
 
So you are saying you checked all the IPs, and none are duplicated? And ran through the possible fixes in the article? The only time I have ever seen a message about a duplicate IP is when there was a duplicate IP. Do you have printers, and other devices that use an IP on the network?