LPC15xx doesn't have Ethernet, so you'd have to add an external MAC & PHY. There are 3rd party Arduino shields that enable this, via the SPI interface, but we don't have example code or recommendations for that at the moment. On the mbed developer site it shows one such board:
https://developer.mbed.org/platforms/LPCXpresso1549/
With the LPCXpresso43xx and LPCXpresso18xx boards they have on-device MAC and a PHY on the board, so you can add the OM13082 general purpose shield to connect Ethernet via RJ45.
