i2c to i2c bridge (router)?
What you are looking for is probably an I2C multiplexer. There is a wide selection of these parts available, but one example is TI's TCA9544A. It's effectively a 1-to-4 switch for I2C signals, which will itself respond to I2C commands at a configurable address. Unlike a normal switch, though, each of the four downstream channels can be switched on or off separately -- so it's possible to enable more than one of those channels at a time, if appropriate, or even to disable them all.

Another option is an I2C address translator. These are less common; one example is Linear's LTC4317. This device can be used to effectively change the address of an I2C device behind the translator.