Okay, so P2P tunnels...
They don't have to carry just channels, although that is one of my primary uses. They can theoretically conduct several things from point A to point B within the ME Network. But let's focus right now on carrying channels, since that's the big use.
In brief: I like to color the cable directly attaching the p2p tunnel itself. This has several benefits: primarily, it color codes my input and output so I know exactly what is going where. It also keeps them from accidentally crossing the streams, for example if you are setting up several around your Controller multiblock.
In effect, consider a p2p tunnel to be an ender conduit through which 32 channels of data may travel through a single pair of channels in an enclosed subnetwork in order to be available on the other end, but not between.
Since that probably didn't explain it any clearer, let me expound upon that a bit and build a giant wall o' text.
By the time you get to the point where you want to play with p2p tunnels, you've got an ME Controller multiblock structure. So we will define your 'primary network' as the one directly attached to that Controller. Anything not directly attached to that is considered a subnetwork. This is going to be an important definition to keep in mind going forward to avoid getting confused.
The need for a p2p tunnel exists when you want to run 32 channels of data to somewhere fairly remote, but that dense cable is expensive. So, instead of running it all over the place, we're going to use a pair of p2p tunnels. One can be directly connected to your ME Controller to provide the maximum 32 channels, whereas the other can be anywhere you need a bunch of channels. In fact, it can be several anywheres, you can have multiple outputs from a single p2p tunnel interface, as long as you have enough channels in your tunnel subnet. So, for example, you have several areas requiring multiple channels who total over eight, which are relatively close proximity to each other, you can keep them all tied into your main network without needing to run bundled cable everywhere.
The 'traditional' use of p2p is to have four sides of an ME Controller hooked up with four different p2p tunnels, the outputs of which then feed into dense cable and are parsed out from there, giving you a total theoretical 128 channels run through a single regular ME cable. However, I've found alternate uses that are less 'efficient' in terms of channel density but more favorable for utility.
For example, I have a wool farm, a cobbleworks, my blood magic area, my thaumcraft area, and my botania area, which are at remote locations from my main base, however they were all within a few chunks of each other. So I ran one p2p tunnel off of my Controller, and each of these locations ended up with enough channels to handle the throughput while eating up only a single 'side' from my Controller. They all fed off of the same 32 channels from that same 'side', exactly as though I had run dense cable all the way out there and then split off, only without paying such a heavy price. Furthermore, since each area had its own p2p tunnel, I didn't have to run a single piece of dense cable anywhere, since each location required less than eight channels.
So, to clarify:
Anything connected to your ME Controller is defined as in your main network. The cable BETWEEN your p2p tunnels, however, is explicitly NOT and should never be hooked up to your Controller, therefore it is an ad-hoc subnetwork. However, the fun part here is that you can, theoretically, add in a Controller to that subnetwork that runs your tunnels, giving you a stupid number of connections possible, and because it never actually connects to your main network, the controllers won't fight each other.
I like to color my p2p cables because I tend to have one directly attached to the Controller that hits several places, so I want to make sure that everyone on the North facing side is, for example, on the 'blue' tunnel network, everything on the East facing side is on Green, and so forth. This helps me keep everything straight. It also lets me know what NOT to connect to. If something is colored, that's my tunnel subnet not my main network, so don't plug into it. Also, I almost always have the 'input' tunnel face directly attached to my controller, which gives it 32 channels without needing any dense cable. However, if you have multiple inputs and outputs, this can get to be very complex, and you may need to document your work with signs.
