Booking a visitor hostel at IIIT Allahabad is simple.
You are only required to send an email to the Deputy Registrar (Finance and Accounts) and put Caretaker Visitor Hostels to CC
The mail should follow the standard format for any official mail and should include your personal details.
However, if you are a prospective student and are likely to take admission for any year, then stating relevant details about the same will be good too. It can entail anything from your JEE roll number to any other particular ID you are expected to submit. Besides, also state your need for accommodation during the period of your admission process.
If you do not receive a response, you can ring any relevant person up using the telephone directory of the institute.