Pokemon Diamond and Pearl: National pokedex
The National Pokedex is something that allows you to access new areas in Sinnoh and migrate Pokemon from Ruby, Sapphire, Fire Red, and Leaf Green to Diamond and Pearl.
In order to get the National Pokedex you must complete two things. You have to encounter the first 150 Pokemon in the Sinnoh Pokedex and defeat the Elite Four. Take note that you only have to encounter
them, you don't have to catch them. This is fairly easy because you will most likely find more than 100 Pokemon by the time you defeat the Elite Four. The only Pokemon you will have trouble encountering are Dialga or Palkia depending on your version. In order to add them to you Pokedex you must visit the old woman in Celestic Town's top house after defeating the Elite Four. She will show a picture of the legendary Pokemon you are missing from the Pokedex. For the locations of all of the first 150 Pokemon in the Sinnoh PokeDex, click here
Once you have met both requirements to get the National Pokedex, go to Sandgem Town and talk to Professor Rowan. He will check your Pokedex and say you've completed it. Professor Oak will come in and give you the National Pokedex. Now you can acess new areas of Sinnoh and trade with the GBA games.