The National Pokédex is an item obtained in the Pokémon video game series. Unlike the normal Pokédex, the National Pokédex allows you to catch/find more Pokémon outside of the 150 Pokémon that can normally be found in the specific region. In each game, obtaining this item is different.
Pokémon Diamond and Pearl
To get the National Pokédex in Diamond and Pearl, you must defeat the Elite Four, and see all 150 Sinnoh Pokémon, excluding Manaphy.