 context already exists), the SGSN does not have to perform all these functions. Instead it re-uses PDP context parameters including the PDP address but excluding QoS parameters. Once activated, all PDP contexts that share the same PDP address and APN are managed equally. At least one PDP context should be activated before a secondary PDP context can be activated for the same PDP address. PDP Address The PDP address is the IP address allocated to a mobile station when it activates a PDP context. It may be an Ipv4 or Ipv6 IP address. The PDP address is associated with the IMSI and it may be permanent or temporarily assigned by the GGSN, DHCP or radius, or by an external public data network (PDN). PDP addresses can be allocated to an MS in four different ways: The home PLMN (HPLMN) operator assigns a PDP address permanently to the MS (static PDP address). The HPLMN operator assigns a PDP address to the MS when a PDP context is activated (dynamic HPLMN PDP address). Core Network Planning and Optimisation The visitor PLMN (VPLMN) operator assigns a PDP address to the MS when a PDP context is activated (dynamic VPLMN PDP address). The PDN operator or administrator (e.g. an Internet service provider or corporate Intranet administrator) assigns a permanent or dynamic IP address to the MS (external PDN address allocation). It is the HPLMN operator that defines in the subscription whether a dynamic HPLMN or VPLMN PDP address can be used. For every IMSI, zero, one or more dynamic PDP addresses per PDP type can be assigned. For every IMSI, zero, one or more static PDP addresses per PDP type can be subscribed to. When dynamic addressing from the HPLMN or the VPLMN is used, it is the responsibility of the GGSN to allocate and release the dynamic PDP address. When an external PDN address allocation is used, the PLMN may obtain a PDP address from the PDN and provide it to the MS during PDP context activation, or the MS may directly negotiate a PDP address with the PDN after the PDP context activation procedure is executed. If t