Minecraft wiki entry for Nether_portals, associated text: You should build portals at 64 Nether block intervals, even if you are not normally going to use these gates. (This is the maximum ideal distance, but they can be built as close to 16 Overworld blocks apart, if the coordinates are accurate.) This is so that if you use Nether portals for long-distance travel, and your usual Overworld destination portal becomes inaccessible for some reason, (due to large gravel cave-ins, lava, water, or you have an automated activation system and forgot to turn it on) you will still have a reasonably close backup gate, which will get you back into your gate network.