National Pokédex
The National Pokédex (Japanese: 全国図鑑 Nationwide 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 the "old Pokédex" in Generation II, while Johto's regional Pokédex at the time was known as the "New Pokédex". 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 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 HeartGold and SoulSilver, everything straightened itself out in the end.
Generation III
Pokémon Ruby and Sapphire
In Pokémon Ruby and Sapphire, the Pokédex is upgraded to the National Pokédex after the game trades with Pokémon FireRed, LeafGreen, or Emerald. Trading with Pokémon Colosseum or Pokémon XD will not unlock National Pokédex.
Pokémon FireRed and LeafGreen
In Pokémon FireRed and LeafGreen, once the player has more than 60 Pokémon owned in the Kanto Pokédex, has completed the Sevii Islands sidequest and has entered the Hall of Fame, if the player talks to Professor Oak in his lab in Pallet Town he will upgrade the Pokédexes of Blue and the player to the National Pokédex.
After obtaining the National Pokédex, the player can obtain Pokémon introduced in Generation II and Generation 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.
Pokémon Emerald
In Pokémon Emerald, after the player has 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/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 to 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, in Littleroot Town Professor Birch will offer the player their choice of one of the Johto starter Pokémon.
Generation IV
Pokémon Diamond, Pearl, and Platinum
In Pokémon Diamond and Pearl, if the player has 150 (or 151, depending if the player possesses Manaphy or not, as Manaphy is optional) Pokémon seen in the Sinnoh Pokédex, they can obtain the National Pokédex (the player does not need to have entered the Hall of Fame). In Pokémon Platinum, if the player has all 210 Pokémon (including Manaphy, as registering it as "Seen" is compulsory this time) seen in the expanded Sinnoh Pokédex and entered the Hall of Fame, they can obtain the National Pokédex.
If the player has met the necessary conditions and talks to Professor Rowan in his lab in Sandgem Town, Professor Oak will arrive and upgrade the player's Pokédex to include the National Mode.
Pokémon HeartGold and SoulSilver
In Pokémon HeartGold and SoulSilver, when the player attempts to board the S.S. Aqua for the first time, Professor Oak gives the player the National Pokédex (this cannot be done until the player enters the Hall of Fame).
Generation V
Pokémon Black and White
In Pokémon Black and White, after the player defeats Ghetsis at N's Castle, when they exit their house in Nuvema Town after resuming their game Professor Cedric Juniper will approach the player, Bianca and Cheren, then upgrade their Pokédexes to the National Pokédex.
Pokémon Black 2 and White 2
In Pokémon Black 2 and White 2, after the player enters the Hall of Fame for the first time, when they resume from their house in Aspertia City Professor Cedric Juniper will visit the player's house and upgrade their Pokédex to the National Pokédex.
Generation VI
Pokémon X and Y
In Pokémon X and Y, after the player has entered the Hall of Fame, when the player enters Lumiose Station Dexio will upgrade the player's Pokédex to the National Pokédex.
Pokémon Omega Ruby and Alpha Sapphire
In Omega Ruby and Alpha Sapphire, after the player has defeated or caught the super-ancient Pokémon, if the player talks to Professor Birch in his lab in Littleroot Town, he will upgrade the player's Pokédex to the National Pokédex.
Because the player can upgrade their Pokédex this early, non-native Pokémon are obtainable before entering the Hall of Fame, and Trainers in areas not accessible until the player can use Waterfall in the overworld will use non-native Pokémon, such as Victory Road's Vito using a Darmanitan.