Users in IPv4 networks typically use only one IP address per interface configured either statically or dynamically via DHCPv4 server. Several techniques can be used to detect violation of that policy. However, IPv6 protocol brings new techniques and possibilities to obtain an IPv6 address. New concepts – autoconfiguration, multiple IPv6 addresses per interface or temporary IPv6 addresses providing privacy for end users introduce new challenges for users identification. Network administrators have to collect additional information for user identification from more sources, e.g. DHCPv6 log, routers neighbor cache, Radius logs, syslog etc. This paper presents analysis of IPv6 address assignment used in current networks together with guidelines how to identify a user in IPv6 networks.