pan no for interstate registration of a business

Yes, same pan is used for registration under GST in different states.......

Sir.,

How to register a firm vertical...?

GST Registration for Branches within a State

Separate GST registration can also be obtained for branches within a state for businesses having multiple business verticals. For GST registration to be issued for branches within a State, the following conditions must be satisfied:

• Have more than one business vertical. Business vertical means a distinguishable component of an enterprise that is engaged in the supply of individual goods or services or a group of related goods or services which are subject to risks and returns that are different from those of the other business verticals.
• The business must not be registered to pay tax under the GST Composition Scheme.
• All separately registered business verticals must pay tax on supply of goods or services to another registered business vertical of the same business and issue a tax invoice for such supply.

Obtaining GST Registration for Branches

GST registration for branches or separate business verticals within the same state can be made in FORM GST REG-01. FORM GST REG-01 is the form used by all businesses for applying for GST registration. Hence, the normal procedure for GST registration application and documents required can be submitted by the business for obtaining GST registration.

After submission of GST registration application, it would be forwarded to an Officer for verification and GST registration certificate would be granted usually within 7 working days. The GST registration certificate would show the principal place of business and additional place or places of business.

Refer:  separate-registration-multiple-business-verticals-within-state