For years, website operators have utilized IP address truncation, the practice of removing the final octet of a user’s IP address, as a method for data anonymization. The belief was that this technique rendered the user anonymous, satisfying privacy requirements. However, rulings by data protection authorities and technical analysis have proven this assumption to be incorrect, establishing that truncation does not constitute true anonymization.
Regulatory Rulings Uncover a Critical Flaw
The practice of IP truncation came under official scrutiny from data protection authorities (DPAs) in Germany. In a widely cited case involving Google Analytics, the Bavarian DPA (BayLDA) determined that using the platform’s `anonymizeIp` function was not sufficient for GDPR compliance. Their reasoning was based on a critical procedural fact: the user’s full, unaltered IP address was first transmitted to Google’s servers. The truncation only occurred after this initial data processing took place. This initial collection and transmission of the full IP address was ruled to be the processing of personal data, regardless of its subsequent modification. This decision established a legal precedent that the process itself, not just the stored result, matters for compliance.
The Technical Reality of Re-Identification
Beyond the legal interpretation, technical evidence confirms that a truncated IP address does not guarantee anonymity. An IPv4 address with its final octet zeroed out still contains the first 24 bits of the original address. This remaining data is often specific enough to geolocate a user to a particular city, region, or the network of a specific company or university. Security researchers have repeatedly demonstrated that this partial IP address, when combined with other browser metadata—such as user-agent strings, screen resolution, language settings, and browser plugins—creates a highly distinctive device fingerprint. This fingerprint has been used to successfully re-identify and track users’ activities across different websites, proving that the remaining data points are sufficient to single out an individual.
These real-world events show that IP truncation is a form of pseudonymization, where direct identifiers are replaced, but it falls short of the anonymization standard required by privacy regulations. The data can still be used to single out an individual with a reasonable degree of certainty, which means it remains personal data under laws like the GDPR.
Concise Cyber 