The VIN for pre 1968 Porsche 911's is located in the front trunk on the passenger side. It is stamped in the panel just in front of the small door that is commonly referred to as the "smugglers box". It is also stamped on a small aluminum plate that is mounted close to the smugglers box (pre-1966) or on the front latch panel (1966 and later).
The VIN for 1969 and later models also appear on a small tag on the driver's side of the windshield that is attached to the front "A" pillar.
Cars manufactured from 1981 onward have a 17 digit number as mandated by international standards. It is located in the same place as earlier models.