The Pokémon Network Center, or Pokémon Net Center for short, is a building in the town of One Island, and is only accessible in Pokémon FireRed and LeafGreen.
In addition to the standard features of a Pokémon Center, it contains the Network Machine, run by Celio, a good friend of Bill. It connects Kanto's, Hoenn's, and Orre's Pokémon Storage Systems, effectively allowing players to link with Pokémon Ruby, Sapphire, and Emerald, Colosseum, and Pokémon XD: Gale of Darkness.
When it is first seen, it is not yet active, but after the player retrieves the 
Ruby and 
Sapphire from Mt. Ember and Five Island, respectively, it begins to function. However, players must enter the Hall of Fame and obtain both the National Pokédex and the 
Rainbow Pass.