I recently replaced a DC that went down suddenly.  After I replaced the DC and expnaded my scope in the process, the follow event log entry is persistent:

Event Type:      Warning
Event Source:      NETLOGON
Event Category:      None
Event ID:      5807
Date:            9/18/2012
Time:            4:00:36 AM
User:            N/A
Computer:      LSDC02
During the past 4.21 hours there have been 340 connections to this
Domain Controller from client machines whose IP addresses don't map to any of the existing
sites in the enterprise. Those clients, therefore, have undefined sites and may connect to any
Domain Controller including those that are in far distant locations from the clients.
A client's site is determined by the mapping of its subnet to one of the existing sites.
To move the above clients to one of the sites, please consider creating subnet object(s)
covering the above IP addresses with mapping to one of the existing sites.  
The names and IP addresses of the clients in question have been logged on this computer in the
following log file '%SystemRoot%\debug\netlogon.log' and, potentially,
in the log file '%SystemRoot%\debug\netlogon.bak' created if the former log becomes full.
The log(s) may contain additional unrelated debugging information. To filter out the needed information,
please search for lines which contain text 'NO_CLIENT_SITE:'. The first word after this string is the client name
and the second word is the client IP address. The maximum size of the log(s) is controlled by the
following registry DWORD value 'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Netlogon\Parameters\LogFileMaxSize';
the default is 20000000 bytes.  The current maximum size is 20000000 bytes.  To set a different maximum size, create the
above registry value and set the desired maximum size in bytes.

For more information, see Help and Support Center at
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