Derive MAC adress from 1wire sensor address.

derive_mac.c

uint8_t mac[]               = {  0x02, 0xAD, 0x3A, 0x6E, 0xFE, 0xE1 }; // DEFAULT MAC

//...
if (sensors.count > 0) {
  // Derive MAC address from OneWire Address
  derive_mac(sensors.sensor[0].address, mac);
}

Ethernet.begin((uint8_t*)mac, ip);

//...

/*
    Derive MAC address from OneWire serial
    by using 6 bit of first byte and last 40 bits
    of w1addr as mac address.
    Bit 7+8 are zeroed to mark mac-address as
    private.
*/
void derive_mac(uint8_t * w1addr, uint8_t * mac) {
    memcpy(mac, w1addr + 1, 6);   
    mac[0] = mac[0] | 0x02;
    return;
}