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Through the loft it will be bare wires, but instead of just having wires dangle from the ceiling I think tucking them into a conduit of some kind will be tidier.
The rooms are 15m apart, and there are 3 brick walls, 1 chimney, and 4 doorways in between. Hence... just go over it all.
The loft is very accessible, so the only thing I really have to do is drill a few holes in the right places.
Velocio-
My dad did it to run coax around so it'll be fine. Just bag up the ends properly so they don't get damaged. I'd have said "watch the length" but 15m is nowhere near the limit .
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...it will be bare wires...
Read a tip somewhere (may have been AV Forums) that suggested avoiding this where possible when routing cables and using conduits all the way as all it takes is an inquisitive rat/mouse/creature and you have to do it all again.
atz
tonylast
Conduit isn't necessary in the loft, although it may be a bit tidier (depending on whether you have flooring up there, and which way the joists are running).
What are your walls covered in - It may be neater to go behind it they're stud / plasterboard.
Routing from top down is also easier than bottom up.
Also, with cat6, you don't need to run the shortest route - signal is good up to 100m, iirc. Meaning you can route wherever keeps it out of the way.