 specifications point of view, an HLR was replaced by an HSS starting from the 3GPP Release-5. However, in practice both systems are still developed and deployed in parallel. There is necessarily no pressing incentive for operators to deploy an HSS unless they also intend to deploy IP Multimedia Subsystem (IMS) and/or EPS. The supported PDN Type is provisioned per each APN for every subscriber. Each subscriber can be provisioned with multiple Access Point Names. The following PDN Types are possible in the HLR/HSS for each APN: IPv4-only PDN/PDP Type - the APN provides only IPv4 services. IPv6-only PDN/PDP Type - the APN provides only IPv6 services. IPv4v6 PDN/PDP Type - the APN provides dual-stack or single IP version services. IPv4_or_IPv6 PDN Type - the APN provides either IPv4 or IPv6 services. This PDN Type is specific to an HSS and does not exist in HLR (i.e. Gr-interface [53]). Hav- ing both PDN/PDP Type IPv4 and IPv6 separately defined for the same APN equals functionally to the PDN Type IPv4_or_IPv6 but depending on the implementation may require duplication of parts of the subscription data. The specific IPv4_or_IPv6 PDN Type brings no additional value in practice. An HLR may have multiple APN configurations of different PDN Types and even for the same name; these configurations would effectively achieve the same functionality. Furthermore, it was clarified in Release-10 that the provisioning of PDN Type IPv4v6 for a given APN means that both PDN Type IPv4 and PDN Type IPv6 are also provisioned for that APN. It should be noted that the PDN Type IPv4v6 does not exist in an HLR, Gr-interface [53] signaling prior 3GPP Release-9. The PDP Type IPv4v6 causes the SGSN either to fail IPv6 in 3GPP Networks the subscription profile download (since the Ext-PDP-Type and Ext-PDP-Address information elements for Gr-interface are not understood) or to treat the PDP Type IPv4v6 as PDP Type IPv4 (see Section 4.5 for a more detailed discussion). IP Transport Plane The user plane IPv6 has no direct impact on the transport 