National Pokédex
The National Mode Pokédex is an upgraded Pokédex that records information from Pokémon around the nation, instead of just ones native to a particular region.
In Pokémon Ruby and Sapphire, the Pokédex is upgraded to National Mode after the game trades with Pokémon FireRed or LeafGreen. Trading with Pokémon Emerald, 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. 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 Dex 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. While players, at that point onwards, may have more than 200 Pokémon caught in their Pokédex, Professor Birch will not give them a choice of the Johto starter Pokémon unless their Hoenn Dex lists 200 caught.
In Pokémon Diamond and Pearl, once the player has at the very least seen 150 of the 151 Pokémon of the Sinnoh Dex 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 Dex list via trades with other Diamond and Pearl games.
National Mode order was known as the "Old Pokédex" in Generation II, while the Johto Dex was known as the "New Pokédex". This arose from the fact that Generation I was the only previous generation. Once Hoenn got its own Pokédex order, the numeration system that started with Bulbasaur had to change its name so as to not reference a region not visitable in Ruby, Sapphire, or Emerald. Being that the Kanto Dex as well went on to merely reference the first 151 entries in National Mode, it seems that all has turned out well.