This specification identifies the routeing scenarios possible for calls between Public Data Networks (PDN's) and the GSM PLMN. As one of the prime means of routeing is based on the subscriber number@ this recommendation initially defines the numbering principles for interworking between PLMN's and PDN's. These principles indicate: - present PDN procedures when aiming at single stage interworking for the near term and - future PDN procedures exploiting the 15 digit and intelligent signalling capabilities of the ISDN as the long term solution. Further routeing scenarios are presented for PDN originated calls to mobile subscribers who are roaming on a foreign PLMN.