The NINO (National Insurance number) is a number used in the United Kingdom in the administration of the National Insurance or social security system. It is also used for some purposes in the UK tax system. The number is described by the United Kingdom government as a "personal account number".
The format of the NINO number is two prefix letters, six digits, and one suffix letter. The example used is typically BN102966C. Often, the number is printed with spaces, like this: BN 10 29 66 C. (wikipedia)
Neither of the first two letters can be D, F, I, Q, U or V. The second letter also cannot be O. The prefixes BG, GB, NK, KN, TN, NT and ZZ are not allocated. Validation lists of issued two-letter prefixes are published from time to time.
After the two prefix letters, the six digits are issued sequentially from 00 00 00 to 99 99 99. The last two digits determine the day of the week on which various social security benefits are payable and when unemployed claimants need to attend their Jobcentre to sign on (renew their claims): 00 to 19 for Monday, 20 to 39 for Tuesday, 40 to 59 for Wednesday, 60 to 79 for Thursday and 80 to 99 for Friday.
The suffix letter is either A, B, C, or D. (although F, M, and P have been used for temporary numbers in the past). The NI number is unique without the suffix letter, so, for example, if AB 12 34 56 C exists, then there will be no other numbers beginning with AB 12 34 56 (although temporary numbers were not necessarily unique, because two people with the same date of birth would have had the same number). In official electronic submissions, the final letter may be represented by a space if not known.
This tool was developed for programmers and testers who have the constant need to enter different NINO numbers in developing forms.