Quoting Erik Anderson <erikerik at gmail.com>:

> On Tue, 15 Mar 2005 09:07:04 -0600, Steve Swantz
> <aintboeingaintgoing at gmail.com> wrote:
>>
>> Since ethernet only requires 4 wires and cat5 comes with 8, can you
>> wire two jacks from one cable, using two twisted pairs to each jack,
>> or are there any interference/performance issues involved?
>
> This is definitely possible, as we have recently had to do this at
> work in a few places.  (don't ask...it's one of the ramifications of
> working in a 100+ year old building).
>
> Anwyays - it works great.  I can't remember the exact pinouts right
> now, but I'll go grab one of the cables we made and get back to you
> with the pinouts.
>
> -Erik
>

While it will technically work, you're kind of asking for trouble. This is the
type of arrangement that can end up with weird network errors that are hard to
reproduce. This type of arragement was more popular, and more feasible, when
people where running 10BaseT ethernet. Fast ethernet depends a lot more on the
properties of the electro-magnetic field created by data moving through the
cat-5 in a specific fashion. Introducing a different field into the mix in the
form of the phone signalling can cause problems.

Josh