[ubuntu] DNS server working but sometimes not for local intranet

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http://ubuntuforums.org – I set up a DNS server on the local network to provide pages for an intranet, the network router was set to use the DNS server as its primary and only DNS server, to ensure that all the local pages would be accessible and they were; all computers on the network could access all the pages without problem, including mobile phones and tablets connected to the wireless extenders. However, today the pages cannot be reliably found, occasionally page could not be found errors appear for local pages but the DNS server is still forwarding external pages correctly... The pages are all stored on the same (Hardware)