Customer ID is a unique identification number assigned to every customer holding savings or current accounts in HDFC Bank. It is a nine-digit number that is required while logging into NetBanking, and it is used by bank officials to identify the customer and their account. The customer ID is printed on the first page of the HDFC bank’s cheque book, passbook, and account statement. If you are unable to locate your customer ID, you can retrieve it online by logging into NetBanking and clicking on the "?" icon. You will need to input your registered mobile number (with country code) and your PAN or date of birth details, and authenticate using the OTP sent to your registered mobile number. The unique customer ID will be displayed upfront. Alternatively, you can obtain your customer ID by visiting any branch of HDFC Bank and asking the bank staff for your customer ID.