The Astral Plane

The various planes of Allenia all travel within the Astral Plane. All the major planes of the universe orbit Allenia, which sits at an intersection of the Astral Plane, the Ethereal Plane, and the Shadow Plane. The Astral Plane touches all the planes, and serves as a means to travel between those planes.
The Astral Plane is a great silvery sea, sometimes describes as a starry sky behind the starry sky of the world. Travel throughout the plane is at the speed of thought. The planes float throughout the Astral Plane, and the astral Plane serves as the conduit for summoners when they wish to summon extraplanar creatures. 
Unknown to most, the Astral Plane does indeed have a border, a boundary of utter darkness called the Outer Darkness. Some believe the Dark Six are trapped beyond the Outer Darkness. No mortal has traveled beyond the Astral Plane, though.

Type

Dimensional plane

Location under

Inner Planes