As your researching tires for your 911, Cayman or Boxster, you may come across N-0 designated tires. Tires may be specified for a particular vehicle or range of vehicles and must successfully pass the tire company's laboratory tests to assure they would be capable of supporting the Porsche vehicle while allowing it to reach its top speed on Germany's Autobahn. Additional tests are conducted to confirm prototype tires meet Porsche's noise, hydroplaning and handling requirements. These tires will also be evaluated to assess their high-speed durability, uniformity and serviceability. Production tires that have passed all the tests and received the engineering department's release can be branded with an N-specification.
Working with the tire manufacturers allows Porsche to have tires on their vehicles that help them steer, brake and accelerates the way they intended. Porsche and the manufacturers also use the tires to tune the chassis' overall understeer/oversteer bias to avoid a fishtail (oversteer) or excessive push (understeer).
That's where the N-Spec designation comes into play. It's critical that you match your front and rear tires so proper handling is retained. On your Porsche, rear tires will typically wear out sooner, so it's common to need replacement tires for the rear axle only. In addition to the brand and model, look on the tire's sidewall to see which N-specification is used on the tire: N-0, N-1, N-2, N-3 or N-4. These markings clearly identify them as approved by Porsche for their vehicles. You'll want to make sure your current front tires match the new rears you're purchasing.
For example, when looking to replace 18" tires on a 2008 Porsche 911 Carrera be sure to check your current tires to verify size as the vehicle also was available in a 19" wheel option. If you have Bridgestone Potenza RE050A N-0 tires on the front with plenty of service life left in them, then look at the matching tire in a 265/40R18 size for the rear.
For more information on Porsche tires, read "Porsche N-Specification Tire Approvals."