National Pokédex
The National Pokédex (Japanese: 全国図鑑 national encyclopedia) is a Pokédex that has been upgraded with the National Mode 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.
National Pokédex order was known as Old Pokédex Mode (Japanese: ふるいずかんモード old-style encyclopedia mode) in Pokémon Gold, Silver, and Crystal, while Johto's regional Pokédex order at the time was known as New Pokédex Mode (Japanese: しんがたずかんモード new-style encyclopedia mode). This arose from the fact that Generation I was the only previous generation, and one could journey to Kanto in Generation II. After Hoenn received its own Pokédex order, however, the numbering system that began with Bulbasaur had to change its name so as not to reference Kanto, itself inaccessible in Pokémon Ruby, Sapphire, and Emerald. Considering that the term "Kanto Pokédex" went on to reference only the first 151 members of the National Pokédex, while the "Johto Pokédex" references the new order for Pokémon HeartGold and SoulSilver, everything straightened itself out in the end.
Generation III
Ruby and Sapphire
The Pokédex is upgraded with the National Mode after the game trades with Pokémon FireRed, LeafGreen, or Emerald. Trading with Pokémon Colosseum or Pokémon XD: Gale of Darkness will not unlock the National edition Pokédex.
FireRed and LeafGreen
Once the player has more than 60 Pokémon owned in the Kanto Pokédex, has completed the initial Sevii Islands sidequest of rescuing Lostelle and has entered the Hall of Fame, he/she may talk to Oak in his lab in Pallet Town so that their and Blue's Pokédexes are upgraded 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 Knot Island to put into the Network Machine.
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, 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 obtained in the Hoenn Pokédex, Birch will offer the player one of the Johto starter Pokémon.
Generation IV
Diamond, Pearl, and Platinum
In Diamond and Pearl, if the player has seen the first 150 Pokémon in the Sinnoh Pokédex, they can obtain the National Pokédex. In Platinum, if the player has seen all 210 Pokémon in the expanded Sinnoh Pokédex, they can obtain the National Pokédex. If the player meets these conditions, they can talk to Rowan in his lab in Sandgem Town. Oak will then arrive and upgrade the player's Pokédex with the National Mode.
HeartGold and SoulSilver
When the player attempts to board the S.S. Aqua for the first time after entering the Hall of Fame, Oak appears to upgrade the player's Pokédex with the National Mode.
Generation V
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.
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.
Generation VI
X and Y
After the player has entered the Hall of Fame, Dexio will upgrade the player's Pokédex with the National Mode in Lumiose Station.
Omega Ruby and Alpha Sapphire
After the player has defeated or caught the super-ancient Pokémon, they may talk to Birch in his lab in Littleroot Town so that he upgrades the player's Pokédex with the National Mode.
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 unlock the National Mode, 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.