In simple words
If you demat the old certificates they (Depository Participant) will issue the DP a/c number i.e. DP Id after the succesfull demat of the certificates (provided the companies should exist and listed in bse or nse),
Its just like a opening a bank account in which shares are deposited instead of money.
Share certificates are returned back in case the companies are de-listed from bse and nse.
You can check the company at
Bombay Stock Exchange - BSE site for SENSEX, stock quotes and market trend and
National Stock Exchange of India Ltd. for its share price (only check these sites as other sites may not show all the stocks).
click on
List Of Companies to see the companies listed category wise in BSE.
You can also check the companies which are renamed in last 10-20 years whose physical certificate you might still be holding.
DP's like Indiabulls and Icicidirect can help you demat as well as open your online trading account for trading online after demat of your physical shares.