100 meters is the maximum Ethernet cable length. But if you then place a switch, you can go for another 100 meters. There is no theoretical limit, only a practical and financial limit. You could use fiber to go longer distances more easily. Or those Ethernet-over-telephone cable converters, they allow for several hundred meters (but $$$).
Another solution could be to use two serial/GSM converters, and go over the GSM network for long distance. But $$$ and monthly fee.
