code: func nodeItemKey(id ID, ip net.IP, field string) []byte {
  	ip16 := ip.To16()
  	if ip16 == nil {
  		panic(fmt.Errorf("invalid IP (length %d)", len(ip)))
  	}
  	return bytes.Join([][]byte{nodeKey(id), ip16, []byte(field)}, []byte{':'})
  }
docs: // nodeItemKey returns the database key for a node metadata field.