Rob,
Thanks for writing, and glad to hear you’re planning to hike the JMT. It’s a wonderful trail.
As for campsite reservations, there are no reserved campsites along the John Muir Trail itself. There are established campsites — these are sometimes noted on the maps, but more than often they are not. An established campsite is usually obvious — and one of the indicators is a fire ring.
You are requested to camp in established campsites (where others have clearly camped before) unless otherwise posted, or on hard ground where you will leave no trace (not on grass or other living material). There are rules about campfires that vary typically by elevation. Usually it’s right around 10,000′ — above which no campfires are allowed. There are usually signs that will warn you of this when you pass that elevation.
Where campfires ARE allowed, you’ve got to use only established campsites and fire rings. Rangers don’t want you building fires where none have been built before because they scar the rocks.
I’m sure you already know this, but it’s worth noting that while you don’t need to reserve specific campsites along the JMT, you DO need to have a permit for your trek. This is issued by whatever jurisdiction you begin your trek in. So if you start in Yosemite National Park (as most do), you’ll get this from the Yosemite ranger station. Your JMT permit is good for the entire journey, even though you pass through multiple parks and jurisdictions.
Hope this helps!
.
