DNS Problem

Hi,

We have Large Network, Domain forest (windows 2003)is in Head office and branch office are in child domain. We have problem in the network if internet is not working due to this problem the branch office some users are not able to browse local intranet sites & SAP also. If we given local intranet site IP address for browsing we able to browse & same way give IP address for SAP then its working. If internet is working afterwards all working with hostname. we are find very difficult to sort the problem. please help to us.