By default a KMS client will cache the KMS host that it was able to activate with and will communicate directly with this host when it is time to reactivate (instead of querying DNS). If the client cannot contact the cached KMS host, discovery with DNS will be used.
You are able to disable the caching with the following command:
slmgr /ckhc (or cscript c:windowssystem32slmgr.vbs /ckhc)
To enable caching again use the command:
slmgr /skhc (or cscript c:windowssystem32slmgr.vbs /skhc)
You can verify if caching is active with the command:
slmgr /dli (or cscript c:windowssystem32slmgr.vbs /dli)
Office 2010 will also cache the KMS server used, but in order to disable this you will need to set a registry key: