We study attempts to dynamically update DNS records for private (RFC1918) addresses, by analyzing the frequency spectrum of updates observed at an authoritative nameserver for these addresses. We developed a binary autocorrelation algorithm and discovered that updates come in infinite series with periods of 60 or 75 minutes. We identify both periods as default settings of out-of-the-box Microsoft Windows 2000 and XP DNS software. Identifying this common property of end-user environments helps to understand users' behavior on the Internet. To our knowledge this is the first study of the global impact of dynamic DNS.