(a) Service at municipality. A motor carrier of property, motor passenger carrier of express, and freight forwarder authorized to serve a municipality may serve all points within that municipality's commercial zone not beyond the territorial limits, if any, fixed in such authority.

(b) Service at unincorporated community. A motor carrier of property, motor passenger carrier of express, and freight forwarder, authorized to serve an unincorporated community having a post office of the same name, may serve all points in the United States not beyond the territorial limits, if any, fixed in such authority, as follows:

(1) All points within 3 miles of the post office in such unincorporated community if it has a population of less than 2,500; within 4 miles if it has a population of 2,500 but less than 25,000; and within 6 miles if it has a population of 25,000 or more;

(2) At all points in any municipality any part of which is within the limits described in paragraph (b)(1) of this section; and

(3) At all points in any municipality wholly surrounded, or so surrounded except for a water boundary, by any municipality included under the terms of paragraph (b)(2) of this section.