From Bulbapedia, the community-driven Pokémon encyclopedia.
The National Mode Pokédex (Japanese: 全国図鑑 Nationwide Encyclopedia) is an upgraded Pokédex that records information from Pokémon around the nation, instead of just ones native in a particular region.
In Pokémon Ruby and Sapphire, the Pokédex is upgraded to National Mode after the game trades with Pokémon FireRed, LeafGreen, or Emerald. Trading with Pokémon Colosseum or Pokémon XD will not unlock National Mode.
In Pokémon FireRed and LeafGreen, National Mode is acquired after the first defeat of the Elite Four, if the player has more than 60 Pokémon owned in the Kanto Pokédex and the Sevii Islands sidequest is done. After this, the player can acquire Pokémon released in Generation II and Generation III via capture or evolution. This also allows Generation I Pokémon to evolve into their Generation II forms (such as Golbat being able to evolve into Crobat). However, to be able to trade with anything besides another copy of FireRed or LeafGreen, Celio's machine on Knot Island has to be completed with both the Ruby and the Sapphire, which will require venturing further south in the Sevii Islands archipelago, where Pokémon not in the Kanto Pokédex can be seen.
In Pokémon Emerald, National Mode is also acquired after the first defeat of the Elite Four, and from that point on may trade with any of the other versions in Generation III provided that they also have unlocked capabilities to trade with it. Professor Birch will also give players a choice of the Johto starter Pokémon if their Hoenn Pokédex lists 200 Pokémon caught, but not if only their National Pokédex does.
In Pokémon Diamond and Pearl, once the player has at the very least seen 150 Pokémon of the Sinnoh Pokédex, (Manaphy is the only Pokémon in the Sinnoh Pokédex in these games that doesn't need to be seen), and talks to Professor Rowan, Professor Oak will arrive to upgrade the Pokédex to include the National Order. This, however, is not required to be able to receive Pokémon not in the Sinnoh Pokédex list via trades with other Generation IV games. In Pokémon Platinum, the requirement is the same, however, all 210 Pokémon in the expanded listing must be seen, including Manaphy. However, Manaphy can now be seen without trading by reading the book at the Pokémon Mansion.
In Pokémon HeartGold and SoulSilver, Professor Oak gives the player the National Pokédex before he/she boards the S.S. Aqua for the first time. However, as with Diamond, Pearl, and Platinum, it is not required to obtain Pokémon not in the games' regional Pokédex (in this case, the Johto Pokédex).
The Pokémon Mew, Celebi, Deoxys, Jirachi, Manaphy, Phione, Arceus, Shaymin, and Darkrai are not needed to complete the National Pokédex.
In Pokémon Black and White, Professor Cedric Juniper upgrades the player's Pokédex to the National mode when the player first leaves his/her own house after defeating N and Ghetsis. As with previous generations, event Pokémon including Keldeo, Meloetta and Genesect are not needed to complete the National Pokédex. In Black 2 and White 2 Cedric Juniper will upgrade the Pokédex in player's house, after (s)he defeats the Champion.
National Mode 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. Once Hoenn got 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.