APN is a character string written in the MS, either by default, by push-configuration or by the subscriber. This name (for instance “net”) would be then interpreted by the GGSN, in order to understand what is needed by the MS.
HTTP ? FTP ? WAP ?
Depending on the required service(s), a certain IP adress can be allocated to the MS.
But ff the APN “string” is not in the GGSN, then I guess the GGSN is not able to allocate a PDP context.