It's called an Eruv. Every major city has at least one eruv that covers several square miles. The line must be unbroken and must have certain types of supports that anchor it to the ground. In reality it's a string they run up along the telephone poles by the wires.

Here's a map of the manhattan eruv, which "encloses" most of manhattan: https://images.shulcloud.com/634/uploads/manhattan-eruv.jpg

For comparison here's the one in northern Dallas: http://i1.wp.com/www.toraschaimdallas.org/wp-content/uploads/2007/03/Eruv.jpg