Two ethernet ports to bridge two different networks

Using Arduino Networking, Protocols, and Devices
10

That router (RB750) and a single Arduino Ethernet (or an Uno/ethernet shield) will do the job. They are about the same price. Actually, the router may be less expensive than an Arduino Ethernet.

However, my favorite is the RB433UAH. It isn't cheap, but it really works good. Not only does it supply the network connection and the bridge, but power from the usb port and a way to connect remotely to the Arduino usb port from the internet/localnet like a telnet connection.
http://routerboard.com/RB433UAH

edit: I use a Mega 2560/ethernet shield combination with the RB433UAH.