The first digit of an IP rating describes its protection from solid objects such as gravel, insects and other dust. The second digit relates to its protection from moisture, which is directly relevant to the bathroom. The higher the number, the more protection it has.
Diagram and explanation of bathroom lighting zones:
Bathroom Zone 0 - the inside of the bath or shower that can hold water. This area has regular exposure to water, or complete immersion. Electrical products must be low voltage (no higher than 12V) and rated no lower than IPX7.
Bathroom Zone 1 - the area directly above zone 0, limited to a height of 2.25m above the bath or shower. This area requires electrical products to have an IPX4 rating or higher.
Products using Safety Extra-Low Voltage (SELV) can be used in this zone, but the transformer must be located in zone 3 or beyond. If the fitting is fed by a 240V supply a 30ma residual current device (RCD) must also be used to protect the circuit in this zone.
Bathroom Zone 2 - the area beyond zones 0 and 1, stretching 0.6m horizontally and up to 2.25m vertically. This also includes the recessed area of a window with a sill next to the bath. Electrical products must have an IPX4 or better. SELV can be used in this zone too, with the transformer located in zone 3 or beyond.
Not Zoned (was Bathroom Zone 3) - the area beyond zone 2. Zone 3 used to stretch 2.4m horizontally and up to 2.25m vertically, but this is now not separated and any lighting fitting can go in this area. There is not a specified IP rating for this area, although some products are marked as not for bathroom use. SELV or shaver units are permitted. However, all other portable electrical items such as hairdryers, floor lamps or extension cables are not.
All Zones (inc old zone 3)
Any electrical circuits in the bathroom must be protected by a 30mA Residual Current Device (RCD).
Electrical equipment exposed to water-jets. e.g. for cleaning purposes, must be at least IP45 (IPX5).