Polarised rj45?

You can get RJ45 (8p8c) connectors with the release tab off to one side...

For 6P6C there was the DEC MMJ, must admit I never saw any 8 pin versions of them.

That very much depends on what the cabling is actually for. If a very good robust connection is needed then it seems to me that orrible crimpable connections are not going to be long lasting. Brian

Lego Mindstorms NXT uses 6p6c:

Sockets and crimper might be harder to come across...

Not quite sure what the OP means by "network" are both ethernet/IP data networks or is the AV one something else. With data networks two or more sub-nets on the same physical infrastructure isn't a problem, just make sure the netmasks and IP ranges don't overlap. Maybe use

Alternatively ethernet up to 100 Mbps only uses two pairs so 6p6c,

Source? Sockets as well as plugs. Are they non interchangeable with standard RJ45, both ways.

The A/V is analogue, with 12V power line.

What do you expect to happen if someone plugs a PC into an AV socket? What signals/power do you have on them?

You use crimpable connectors because they are more reliable and last longer brian.

So as long as you connect the 12V to different pairs on the ethernet then nothing bad will happen if you plug a PC in. The signal pairs on ethernet are isolated by pulse transformers with a minimum 1500V isolation. You just don't want to apply 12V to either side of one of the transformers so putting it down different pairs will be fine. Power over ethernet does just that so you could stick to the same pinout for the power.

Sorry, my bad - I jut looked at the one I have and its 6p6c not 8.

That's worth considering. The network is wired as 2 x 4p4c

Ah, hopefully sensibly wired unlike some kit I used last year. NIC's don't like 48 V across 3 & 6 (and 4 & 5 (if they are gigabit). Large red warning labels on our cables seemed to prevent mishaps.

That's +V on both wires of one pair and 0 V on both wires of another. I'd be tempted to use 4/5 and 7/8 as those are unused for until you go to gigabit ethernet.

That would be sensible but you still have to be wary of uncontrolled power on Cat5 cables in case you plug something in that doesn't have transformer isolation, like a cheap flying spot "network" tester.