Every device connected to Ethernet has to have a globally unique address, including our network connected SplashLight, it’s called a MAC address. A standard MAC address is 6 bytes, the first 3 bytes are the OUI (Organizationally Unique Identifier). The IEEE, who control OUI assignment, let you register your own OUI for $1750. The final 3 bytes of the MAC can be anything you want, giving you 16 million addresses. Alternatively they also sell IABs (Individual Address Block) for $600, these are addresses are intended for people who can’t afford an OUI or don’t need that many addresses. IEEE give you 4096 address from an OUI they own. So you have control over the last 12bits.
So do you have to spend a lot of money? well no there’s another way. Microchip sell an SPI/I2C eeprom with a unique MAC address pre-programmed in to it already. The 25AA02E48 costs only 0.30p from Farnell. For the small batches we plan to produce this is best option and it means there’s an eeprom on board for us to save our non-volatile data to.