Sort codes, in the British and Irish banking industries, are bank codes used to route money transfers between banks within their respective countries via their respective clearance organisations. In Ireland, a sort code is known as the NSC or National Sort Code and is regulated by IPSO (Irish Payment Services Organisation). Although sort codes in both countries have the same format, they are regulated by different authorities as each country has its own banking system.
Banks in Northern Ireland can be part of either the British or the Irish clearing system, depending on their country of registration. Also, sort codes for Northern Ireland branches of banks registered in both Northern Ireland and the Republic, are valid, and recognised for use in the Republic. The numbering ranges for both are complementary, but do not overlap.
The sort code, which is a six-digit number, is usually formatted as three pairs of numbers, for example 12-34-56. It identifies both the bank and the branch where the account is held. In some cases, the first digit of the sort code identifies the bank itself and in other cases the first 2 digits identify the bank.[1] Although there is a strong correlation between BIC Codes and sort codes, sort codes are not explicitly encoded into BIC codes (although they are encoded into IBANs).
Source: wikipedia.org