The IP rating standard gives an indication of protection of ingress from foreign object (first figure) for example dust, and the second figure gives an indication of protection of ingress of water. see below. 

Although the manufacturer can design the enclosure to meet the standard, the elements that are in the control of installer include how you fix the camera to the wall/ceiling and how the cable entry is managed whilst still maintaining the IP rating. Normally this will include sealing screw holes (or using the correct brackets) and using the correct cable glands. 

IP Rated Enclosures – quick find chart 

First Digit (intrusion protection) 

0 No special protection 
1 Protection from a large part of the body such as a hand (but no protection from deliberate access); from solid objects greater than 50mm in diameter. 
2 Protection against fingers or other object not greater than 80mm in length and 12mm in diameter. 
3 Protection from entry by tools, wires etc, with a diameter of 2.5 mm or more. 
4 Protection against solid bodies larger than 1mm (eg fine tools/small etc). 
5 Protected against dust that may harm equipment. 
6 Totally dust tight. 

Second Digit (moisture protection) 

0 No protection. 
1 Protection against condensation. 
2 Protection against water droplets deflected up to 15° from vertical 
3 Protected against spray up to 60° from vertical. 
4 Protected against water spray from all directions. 
5 Protection against low pressure water jets (all directions) 
6 Protection against strong water jets and waves. 
7 Protected against temporary immersion. 
8 Protected against prolonged effects of immersion under pressure