How to recover virtual machines back in case of virtual machine failure

When you create a snapshot, all changes made on the initial virtual disk image are not actually made on the image itself, but they are written to a new (snapshot) disk file. This action is so fast because there is no need to copy whole virtual disk image, because it works on copy on write principle (only changed, i.e. written blocks are written to snapshot image). Note that snapshot image grows as you change more and more data on your original virtual disk image (which stays as it was on the moment you took the snapshot). It will most probably be much smaller than original image, but in the worst case it will be exactly the same size (if all blocks were changed).

There are two actions you can make with this new snapshot image:

1. Discard snapshot: very fast action. Virtual machine manager simply deletes snapshot image file and returns control onto the original disk image or some of the previous snapshots
2. Merge snapshot: virtual machine manager merges changed blocks (i.e. snapshot image) with original image. This action will take time depending on how much blocks from original image have changed. Again, worst case scenario is that whole original image will be overwritten in case that all blocks from original image have been written after creation of a snapshot.

All procedures described above also work for multiple snapshots. In that case original image may be one snapshot, and next snapshot may reference block on that (first) snapshot. This way you can have many snapshots you can discard or merge with ease.

There are actually 4 sets of Locally Administered Address Ranges that can be used on your network without fear of conflict, assuming no one else has assigned these on your network:

x2-xx-xx-xx-xx-xx
x6-xx-xx-xx-xx-xx
xA-xx-xx-xx-xx-xx
xE-xx-xx-xx-xx-xx

Replacing x with any hex value.