I am told by the gurus here that messing with your cable length can cause your connection to get messed up. I'd make sure you get full info first. Take a look here:
http://www.cableforum.co.uk/board/12...able-wire.html
It doesn't give you the full details but it does give you an idea of the kind of issues you need to research.
If at all possible it is better to leave the modem cable alone and get a longer ethernet cable instead. Is that really out of the question?
Nice nickname you've got there.

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