Windows caches DNS responses to speed up network access, but sometimes this can cause a problem. Positive responses (i.e. successful lookups) are cached for 24 hours, and negative responses (i.e. failed lookups) for 5 minutes.
If you make changes to DNS and want to test the results straight away, you need to clear the cache with:
You can view the current cache with:
ipconfig /displaydns | more
to see a screen at a time