The National Pokédex is a Pokédex that records information from Pokémon around the nation, instead of just ones native in a particular region. The National Pokédex is not available at the start of the games, but it can be unlocked by completing certain tasks.
The National Dex was simply known as "Old Pokédex Mode," while Johto's regional Pokédex was known as "New Pokédex Mode."
Pokémon Ruby & Sapphire
Pokémon FireRed & LeafGreen
In Pokémon FireRed and LeafGreen, if the player has 60 or more Pokémon owned in the Kanto Pokédex, has rescued Lostelle from the Berry Forest and has entered the Hall of Fame, upon talking to Professor Oak in his lab in Pallet Town, he will upgrade both the player's and Blue's Pokédexes with the National Mode.
After obtaining the National Pokédex, the player can obtain Pokémon introduced in Generations II and III via trade and evolution. This allows Generation I Pokémon to evolve into their Generation II forms (such as Golbat being able to evolve into Crobat); prior to obtaining the National Pokédex, these Pokémon will try to evolve but fail. However, the player cannot trade with Pokémon Ruby, Sapphire, and Emerald until they have delivered the Ruby and Sapphire to Celio in the Pokémon Network Center on One Island to put into the Network Machine.
In Pokémon Emerald, after the player enters the Hall of Fame for the first time, when they exit their house in Littleroot Town after resuming their game, Professor Birch and Brendan or May will be waiting for the player. Birch will escort the two of them to his lab, where he will upgrade both of their Pokédexes with the National Mode.
After obtaining the National Pokédex, the player can trade with any Generation III game, as long as that game has the ability to trade with Emerald. After obtaining the National Pokédex, if the player has 200 Pokémon (all except Jirachi and Deoxys obtained in the Hoenn Pokédex, Birch will offer the player their choice of one of the Johto Starter Pokémon.
Pokémon Diamond, Pearl, and Platinum
In Pokémon Diamond and Pearl, if the player has seen the first 150 Pokémon in the Sinnoh Pokédex, they can obtain the National Pokédex. In Pokémon Platinum, if the player has seen all 210 Pokémon in the expanded Sinnoh Pokédex, they can obtain the National Pokédex. If the player has met the appropriate condition for that game and talks to Professor Rowan in his lab in Sandgem Town, Professor Oak will arrive and upgrade the player's Pokédex with the National Mode.
Pokémon HeartGold and SoulSilver
In Pokémon HeartGold and SoulSilver, after entering the Hall of Fame, when the player attempts to board the S.S. Aqua for the first time, Professor Oak will appear and upgrade the player's Pokédex with the National Mode.
Pokémon Black and White
In Pokémon Black and White, after the player defeats Ghetsis at N's Castle and exits their house in Nuvema Town after resuming the game, Cedric Juniper will approach the player, Bianca, and Cheren, then upgrade their Pokédexes with the National Mode.
Pokémon Black 2 and White 2
In Pokémon Black 2 and White 2, after the player enters the Hall of Fame and resumes the game from their house in Aspertia City, Cedric Juniper will visit the player's house and upgrade their Pokédex with the National Mode.
Pokémon X and Y
Pokémon Omega Ruby and Alpha Sapphire
In Omega Ruby and Alpha Sapphire, after the player has defeated or caught the super-ancient Pokémon, if they talk to Professor Birch in his lab in Littleroot Town, he will upgrade the player's Pokédex with the National Pokédex.
Unlike most games, non-native Pokémon are obtainable without the National Pokédex; they begin appearing as soon as the player has met the prerequisites to obtain the National Pokédex, regardless of whether they obtain it. Using the PokéNav Plus to attempt to view their Pokédex entries will result in an entry of red text saying that no information is available on the Pokémon, and the areas found display will show "No reported sightings" over the map.
Pokémon Sun and Moon
Pokémon Sun and Moon does not include the National Pokédex.
As part of its version 1.3 update to be compatible with Generation VII, a National Pokédex feature was also added to Pokémon Bank. Bank's National Pokédex is able to display Pokédex entries from any Generation VI or later game. The Pokédex is updated based on the data of any game that is used to connect to Pokémon Bank.