Hello Miguipda,
I'm not familiar the Asus WL-500gP router, anyway I'm afraid getting your project done will not be so easy as connecting a DS18B20 into any of the router ethernet ports...
However, I think it might be possible to acomplish it in one of two ways.
But, before anyting else, please keep in mind that any router's RJ-45 ports are intended for ethernet only. If you connetct anything else (like the DS18B20) to it you can risk to burn something, so please don't do it :-)
Option 1)
My home automation projects rely on routers GPIO's, which are "general purpose Input/Output" pins that you can control its state with software ( http://en.wikipedia.org/wiki/General_Pu … put/Output ).
The OpenWrt W-1 packages mentioned in the above posts implement the 1-wire protocol in one of these GPIO's. That's where I connect the DS18B20 (which is, as you know, a 1-wire device), and where I suggest you to connect.
The drawback of this is that you'll need to open your router and identify one available GPIO. Again, I don't know the WL500gP router and therefore don't know if there is any available (you have to check the forum for HW details on your router. In alternative you may want to open a new post asking some help in finding it). Sometimes it might be needed to use a GPIO that can be feeding a router LED (GPIO's are frequently used to feed status LED's).
Option 2)
When checking the site you referenced I could notice other possibility: The DS18B20 "talks" 1-wire, right? I believe the site has a 1Wire-RS232 converter (I'm not so good at french - http://rosset.org/linux/temperature/howto2.html ). Then, you just need a cheap RS232-USB converter. Maybe something like ( http://www.ebay.com/itm/USB-2-0-RS232-S … 2316800dce ).
In order to choose the RS232-USB converter, you may need to choose a chipset that is supported by OpenWRtT. Again, you can check the forum and try to find some information about it.
You'll have to check the feasibility for this option. Searching and asking help in the forum might be necessary.
This option can give you more work, but at least you don't need to open your router.
So, answering your questions:
1a) No, you can't. Please keep in mind that any router's RJ-45 ports are intended for ethernet only. If you connetct anything else (like the DS18B20) to it you can risk to burn something, so please don't do it :-)
1a I-II) You can have as many as DS18B20's in the same bus as you want. You can connect them in paralell with the same pinout because the 1-w bus is designed for it. However, you'll have to connect it to a proper pin (try to find a GPIO in your router and then load the software mentioned above). It will not work if you connect to a RJ-45 port!
1b)You can connect the DS18B20 directly into any GPIO port (don't forget the pull-up resistor). There is no connector/adapter that translates 1-wire to Ethernet, so you can't really use the RJ-45 ports for the DS18B20.
1c)As I mentioned in the option 2, you can try to have a 1-wire <-> RS232 <-> USB connetction. As you know, almost all USB devices need a driver to work properly. In this case you'll need to find a RS232-USB adapter that has it's driver implemented in the OpenWRT.
I know that probabily this wasn't the answer you were expecting, but I't better you know this in advance rather than risking damaging your material... Keep in mind that it will be possible. The thing is that it will be more challenging :-)
Good Luck!
Jabss