Mac OSX uses a DNS cache. This cache stores the nameserver responses. While this is useful in general cases, it can be a pain when testing a website that is being moved to a different server. The cache will still serve the old ip address of the site, even if the DNS server already has the new ip address configured. To overrule the cache, and request the latest information with the authoritive DNS server, you can flush the DNS cache. To do this, open a Terminal window and enter the following command:

$ sudo dscacheutil -flushcache