Only short Ethernet.begin(mac); works

If I use Ethernet.begin(mac, ip, gateway, gateway, subnet); it does not work, but this shortened form using DHCP? works. Is the Windows the problem. I use Windows 7 pro and a 4 port bridge, my ADSL modem is a DHCP server too.

What are the network settings on the ADSL router's localnet interface? IP and subnet should be enough.

What ip do you get when you use dhcp?

Arduino got always http://192.168.100.16/ for its IP. The modem uses NAT, but I could not find any other settings for NAT or local LAN.

Then use these settings in the Arduino. Hopefully there is not another device using that ip. You could try pinging 192.168.100.2 before assigning it to the Arduino just to check.
ip - 192.168.100.2
dns = 192.168.100.1
gateway = 192.168.100.1
subnet = 255.255.255.0

If those don’t work, compile and run this sketch and post the results.

#include <SPI.h>
#include <Ethernet.h>

byte mac[] = {  
  0x00, 0xAA, 0xBB, 0xCC, 0xDE, 0x02 };

void setup() {
  Serial.begin(9600);

  // start the Ethernet connection:
  if (Ethernet.begin(mac) == 0) {
    Serial.println("Failed to configure Ethernet using DHCP");
    // no point in carrying on, so do nothing forevermore:
    for(;;)
      ;
  }
  Serial.print("IP address: ");
  Serial.println(Ethernet.localIP());

  Serial.print("gateway address: ");
  Serial.println(Ethernet.gatewayIP());

  Serial.print("subnet: ");
  Serial.println(Ethernet.subnetMask());

  Serial.print("dns address: ");
  Serial.println(Ethernet.dnsServerIP());
}

void loop() {
}

The example from IDE works with the settings I got. Today DHCP seems to like different address than yesterday. The result from serial monitor. IP address: 192.168.100.20 gateway address: 192.168.100.1 subnet: 255.255.255.0 dns address: 192.168.100.1

Does this mean that you should not mix fixed IPs and DHCP.

Does this mean that you should not mix fixed IPs and DHCP.

No, it means you should use a static ip assignment outside the dhcp range. You really need to check in your router setup to determine what ip range is being issued, then avoid those.

What make and model router do you have?

My modem is Bewan ibox IAD ARV4510NPW. If you havn't noticed, you have also written to the "Re: Netserver example is not working here with the Duemilanove" thread. It is also mine. About the same problem but different angle.

The problem with my modem is that there is no setting for DHCP/NAT range, at least not locally. I will check via ISP netsite sometime, because there are some settings there too..

With your router, login and select "Basic setup" on the menu. Then select "LAN TCP/IP and DHCP server". The settings should be in there. The manual I viewed online was not very technical, so I do not know what that page will look like, but it should have all the network settings there.

I wish I could but my ISP has their own Firmware in the box, and it seems I can't access those settings.

Then you should contact you ISP and find which ips you can use for static localnet ips.