The Spider Farm is a cave spider XP farm built under a base formerly belonging to snaomi (inactive user). Currently, FallerofFalls has been gifted the snaomi claims. It is marked on Dynmap at x=974, z=360. This spider farm is located right next to a zombie XP farm. There are two ways to reach this location. The first is to sail on a boat from Spawn Island east to the marker on Dynmap. There, enter the house that says "Welcome to snaomi's place." Go down the ladder to the basement floor, and from there, go down the ladder saying Nether portal Zombie spawner." At 974, 44, 360, the ladder reaches the Nether Portal room, which is the second way to enter the spider farm.
After going down the ladder all the way, you reach an intersection showing the way to the zombies and the way to the spider farm. From there, go to the spider farm, not the spider room. Go down the ladder. This room is where to collect spiders and contains an enchantment table with a library. The room also has a switch which damages cave spiders until they only have 1 heart left.
The contraption to hold the spiders away from the player allows the player to melee attack the spiders between a set of iron bars and glass, which the spiders cannot escape out between. However, standing too close will allow the spiders to attack the player back, so standing back while harvesting is advised.
The nether portal is at 979, 44, 361 in the Overworld. In the nether, it is at 121, 14, 46. Both portals are indicated on the Nether Network marker set. It is housed in a cobblestone building with a horse (named Sanya) in a pen outside of it. To reach this nether portal from the nether hub, first go up to the Old Nether Network floor. From there, take the east tunnel located at 23, 13, 45. You will come across the cobblestone building and be able to enter the spider farm from the portal room.