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Is there any way so an ethernet shield to ask the router for an available ip address? For example to ask for an ip an the router to return 192.168.1.2 if it is available. I am asking it because if we set the ip programmatically and it is attached to another device we gonna have problem.

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Using the DHCP (Dynamic Host Configuration Protocol) you can obtain an IP address dynamically from a DHCP server (which a router usually also is), without having to hardcode it into the firmware of the Arduino, thus enabling it to work on different networks.

There is also a library for that: http://www.arduino.cc/en/Tutorial/DhcpAddressPrinter. It's an extension of the official Arduino Ethernet library, written for the Arduino Ethernet Shield, but you'll likely find implementations of DHCP in many Ethernet libraries (I am using the EtherCard library) because it's an important feature to be able to connect to a network without a statically assigned IP and "only" requires UDP (on top of IP).

For e.g. the Ethernet library you call the begin function either with only the MAC address of your device or with MAC and IP address:

Ethernet.begin(mac);
Ethernet.begin(mac, ip); 

In the first case it will use DHCP to obtain an IP address dynamically, in the second case it will use the static IP address you provide. Be warned that the second case, due to the needed implementation of DHCP, will increase the size of the compiled binary significantly.

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