From Bulbapedia, the community-driven Pokémon encyclopedia.
The Universal Pokémon Network was a fan site that operated from 1999-2001. It is now defunct. Circa 2002 the Ultimate Pokémon Network was created as its replacement. During the heyday of Pokémon in the late 1990s and early 2000s, UPN housed an extremely active fan community.
The site existed at the domain http://www.upnetwork.com/ but since defuncting the domain has been taken by a domain squatter.
Together with The PokéMasters, Pokémon Abode and Bulbagarden, Universal Pokémon Network formed a partnership known as the Anti-Spammer Alliance from 1999-2000. After the dissolution of these websites' original communities, the group became retroactively known as the Big Four, as they were generally considered to be the largest, most active, and best ran Pokémon forums of their day.