Thanks a lot!

If you’ve referenced these symbols by name in the body of the form, you already knew you needed them,

Yes, but I do not know how many of them. The purpose of this macro is to create a lexical environment to test a network code. Some functions take just one port as a parameter, but the others - two and more. With this macro it would be very easy to call these functions and to link their IO through the port numbers:

(with-free-ports 0 10
  (fn-1 port-1 port-2 port-3)
  (fn-2 port-1))

And with the first posted version of with-free-ports I achieved it. The problems appeared when I tried to bind start in run-time. From your reply and from the u/lispm comments I understood that this is not a good idea. What I do not understand, is how to use such macros inside another macros/functions, which can supply with-free-ports with start and end parameters ? Also, I cannot figure out, when exactly macro-expansions take place if I use many enclosed macros. Are they expanded all at once when I compile my code?

Anyway, I am very grateful for your suggestions. This helps a lot to learn CL.

Thank you for your reply.

The compiler generally will not execute the LET and then compile the WITH-FREE-PORTS form with the new binding

Here is an example when the compiler does exactly that:

(defmacro with-free-ports (start end &body body)
  `(list ,start ,end ,@body))

(let ((start 1)
      (end 3))      
  (with-free-ports start end NIL))

So, why does it work?