1) I've read in a book that you need an external power source for the ethernet shield (not USB), but I've also seen people using it without that. Do I need one or not?
Ethernet shields work fine while powered from the Arduino board.
Including power from standard USB ports.
Perhaps you mixed something up with GSM shields, which draw a lot more of current.
Or perhaps you refer to some non-standard USB ports:
There are some devices on the market, that are not able to provide 500 mA default current of the USB standard. Such devices as "netbook PCs", "tablet PCs" and even some "notebook PCs". The sometimes provide only 100mA or 200mA instead of 500 mA. In that case you have such a weak USB port with a mobile device, you perhaps can run into trouble.
But you never get in trouble with the Ethernet shield using a standard USB port which provides 500 mA.
2) I'm using an unofficial ethernet shield so it doesn't have a sticker with mac address (it's got a sticker wich says HR911105A, but thats not the mac adress right?). How do i find a mac dress? I've read in a couple of tutorials that it should work to "make up a unique address". How can I make up a unique one?
The MAC address used must be unique to your own local network only. If you have just one Ethernet shield, you can simply use the MAC address given in the example programs of the Ethernet library. If you have more than one Ethernet shield, modify it to different MAC address. There is just one limit: In your LAN each device must have a different MAC adress, so don't re-use any MAC address of any device that's in your LAN!
3) Same problem with the IP address. How do I find one? Ive got a program called Angry IP Scanner, wich generates a list of IP adresses, can I use any of these? (if they aren't used)
If your router is "DHCP enabled", your sketch could use DHCP to get an IP address dynamically. The router will know which IP adresses in your network are unused and provide an IP address your sketch can use. If you use a fixed IP address instead, you better use an IP address that belongs to your home LAN, but is not in use in your home LAN. The "fixed IP address" you select must
- belong to your home LAN
- has not to be used by any other device in your home LAN