A DEMAT Account Number is your personal identification number assigned to you by the Depository Participants with which you have opened your account. This Demat account number is a unique 16-digit account number assigned by the Depository Participant to the demat account holder to carry out trading through electronic mode.

The format of demat account varies between CDSL and NSDL depending on the one who assigns the particular demat account number. For CDSL DEMAT is a 16 numeric character, while in case of NSDL the first two characters are alphabetic (IN) followed by a 14 numeric character code.

For example,

The demat account number in CDSL can be 0123456789012345

The demat account number in NSDL can be IN01234567890123.

Demat account number is also known as Beneficiary ID in case of CDSL.
Each Demat account is a unique code.

A depository participant is basically an intermediary. There are two types of people, one who are investors and others who are depositories.

Any broking firm registered with SEBI can act as a depository participant.

The demat account number is a number allotted to the investor when the demat account is opened whereas the DP ID is an identification number meant for the DP.
How are the DEMAT Account Number and DP ID is connected ?

Within your DEMAT Account Number of 16 digits, the first 8 digits are the DP ID. Thus anyone who has opened a DEMAT account with your stock broker, these first 8 digits will stay the same.

The last 8 digits of your DEMAT account Number are what basically make your client ID.
This is your identification with this Depository Particiapants.

Relation between DP ID and demat account number.
Demat Account Number= DP ID+ Client ID
The Client ID is the ID which identifies the investor’s portfolio.

Example:

In the case of CDSL, your demat account number is 1234567890123456, then:
DP ID is 12345678
Client ID is 90123456

In case of NSDL, your demat account number is IN12345678901234, then:
DP ID is IN123456
Client ID is 91234567
The Client ID remains the same as long as you maintain your demat account with the same demat service provider.