The Nether Network is a network of tunnels and nether portals at low y co-ordinates in the nether, providing fast transit between places without the expense and permanence of a teleport. At the centre of the nether, as a counterpart of Zero Zero and Spawn Island, is the Nether Hub. There are many ways to access the Nether Network, as almost every build of any significance has a connected portal.
While the Nether Hub is currently unpaved, all paths out from it are paved with slabs to prevent mob spawns. Most paths in the network are also >3 blocks high and >2 blocks wide to allow for fast travel by horse. When these paths transition to smaller and unpaved paths they are fenced and sealed with doors to keep zombie pigmen off the busier roads.
Many portals are accompanied by horse pens. The design is such that a player can jump or walk over the fence when on a horse or on foot, but horses cannot escape alone.
The Nether Network started when fallerOfFalls dug a tunnel through the nether between their base and spawn, so that they didn't have to go by boat every time. That tunnel is now called Diagon Alley. Tunnels were then dug off Diagon Alley to others' bases. and well-used tunnels were expanded to be large enough for horses. Later, after the creation of the Nether Hub, many of the tunnels were paved with slabs to prevent Zombie Pigmen and other mobs from spawning.