Belgian Ondernemingsnummer

What it covers

Returns a valid Belgian Ondernemingsnummer (enterprise number) with a correct `97 − (first8 mod 97)` check. Use `format=vat` for the BE VAT number. Set `invalid=true` for a number with a deliberately wrong check.

How it’s calculated

The Belgian enterprise number's two trailing check digits are 97 minus the first eight digits taken modulo 97, zero-padded to two digits.

Payload = first 8 digits = 04203040, i.e. the integer 4203040.
Compute 4203040 mod 97: 97 × 43330 = 4203010, and 4203040 − 4203010 = 30, so r = 30.
check = 97 − r = 97 − 30 = 67.
67 is already two digits, so no zero-padding is needed; check field = '67'.
Full number = 04203040 followed by 67 = 0420304067.

Edge cases

• Leading digit is only ever 0 or 1 — the only issuance blocks the Belgian registry uses; a numeric parse of a 0-leading number silently drops to nine digits.
• Because check = 97 - (n mod 97) lies in 1..97, every 8-digit prefix yields a valid two-digit check, so no prefix is 'dead' and no redraw is ever needed (unlike mod-11 schemes).
• A check value of 00 is unreachable (it would require 97 - (n mod 97) = 0, i.e. n mod 97 = 97, which is impossible); the smallest possible check is 01.

Gotchas

• The modulus is applied to the payload directly (n mod 97), NOT to the IBAN-style ISO 7064 MOD 97-10 variant (98 - (n*100 mod 97)) that lives in the same file — this generator calls the plain mod97, not iso7064Mod9710CheckDigits.
• The check is subtracted from 97, not 98; using 98 would be the wrong (ISO 7064) formula.
• Leading zeros in the 8-digit payload matter for the string representation but are numerically ignored when computing the modulo.