There's something to be said for changing:
// this must be unique
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
To:
// The mac must be unique.
// See the MAC sticker on your board, or make something up that doesn't clash with other devices on your network.
// If you have multiple boards, make up a different set of hex numbers for each one.
byte mac[] = { 0x??, 0x??, 0x??, 0x??, 0x??, 0x?? };
That way you are forced to think about it.