Accurately mapping Internet address space to organizations is critical to understanding the Internet’s organizational ecosystem. Tra- ditional approaches, which rely on individual WHOIS queries often suffer from unclear ownership structure of IP addresses and inconsistent organization names, resulting in ambiguous inferences. Alternative methods that map BGP prefixes to Autonomous Sys- tems Numbers (ASNs) and ASNs to organizations are also inaccurate since ASes often originate prefixes on behalf of their cus- tomers. This paper introduces Prefix2Org, a comprehensive prefix- to-organization mapping framework. We introduce a taxonomy for the holders of IP addresses and a methodology to map IP ad- dresses to organizations, based on the operational rights over them. We develop string processing heuristics and leverage RPKI Certifi- cates and routing data to address inconsistencies in organizational names and aggregate prefixes under unified management. Our pub- lic dataset covers 99.96% (99.99%) of IPv4 (IPv6) prefixes. We validate 9.3% of routed IPv4 addresses with a 99% recall, and 5.6% of IPv6 prefixes with a 99.34% recall. For the two large organizations where we obtained complete ground truth, Prefix2Org produced no false positives. Finally, in two case studies, (i) we characterize organiza- tions that hold address space without an ASN and (ii) demonstrate how RPKI adoption measured through Prefix2Org differs from the previously used AS-centric view.