Sikkim, due to its contiguity to the borders of neighboring countries like Bhutan, China and Nepal, comes under restricted area regime. And so, foreigners are required to obtain Inner Line Permit (ILP) to visit this beautiful state.
It is free of cost and just takes less than twenty minutes if you have proper documents like the photocopies of your passport and visa along with the original and two of your recent passport-sized photos. The validity of the permit is 30 days which can be extended further for 30 days by the approval of the state Government. The extension can be acquired from the FRO (Foreigners' Registration Office) at Gangtok and office of Superintendent of Police at District Headquarters. Sikkim Tourism Offices are at New Delhi, Kolkata, District Magistrate's Office of Darjeeling, Siliguri, and Rangpo.
The Inner Line Permit (ILP) can also be obtained by filling up an online application form, though it would be considered as provisional Inner Line Permit (ILP) unless you get the stamp on it at any of the border check posts of Sikkim.
For the nationals of Pakistan, China, Myanmar, the formalities for acquiring the Inner Line Permit (ILP) get little more elaborate, as they have to get a written approval from Ministry of Home Affairs, Government of India, New Delhi before the Restricted Area Permit can be issued.
Protected Area Permits (PAP) is required by those who are interested in going for trekking in the interior region of the state (Both Indians and Foreigners). It is issued by Tourism and Civil Aviation at Gangtok for specified areas, and is not given to a solo foreigner tourist. There has to be a group of more than one foreigner, who's trekking programme is arranged by a travel agency registered under the Sikkim Tourism Department.
Note: For Nathula Pass, the permit is only given to Indians.