I will preface this by saying in general I don’t think the approach of interning symbols and binding them at runtime is a good idea. If you’ve referenced these symbols by name in the body of the form, you already knew you needed them, so why not just just use LET to create the binding? Without knowing the details of your use case it sounds like it could be an XY problem.

However, this being Common Lisp, there is a way to do most things and the special operator PROGV is used to create dynamic variable bindings. This is not a tool I reach for often, but it does occasionally come in handy. I am not proud of this:

(in-package #:cl-user)
(ql:quickload "alexandria")

(defmacro with-free-ports ((start end &key (prefix "PORT-") limit) &body body)
  (check-type limit (or null (integer 1 *)))
  (alexandria:with-gensyms (first-port last-port index vars values symbol-prefix)
    `(let ((,first-port ,start)
           (,last-port ,end)
           (,symbol-prefix ,prefix))
       (check-type ,first-port (integer 1 *))
       (check-type ,last-port (integer 1 *))
       (assert (>= ,last-port ,first-port) (,first-port ,last-port) "port range not ordered")
       (check-type ,symbol-prefix (or string symbol character))
       ,(when limit
          `(when (>= (- ,last-port ,first-port) ,limit)
             (error ,(format nil "WITH-FREE-PORTS limit (~A) exceeded" limit))))
       (loop named with-free-ports
             for ,index from ,first-port to ,last-port               
             collecting (alexandria:symbolicate ,symbol-prefix (princ-to-string ,index)) into ,vars
             collecting (format nil "127.0.0.1:~A" ,index) into ,values
             finally (return-from with-free-ports (progv ,vars ,values ,@body))))))

To use it:

(let ((start 80))
  (with-free-ports (start (+ start 5) :limit 10)
    (setf port-80 "http")
    (values port-80 port-85))) ;; => "http", "127.0.0.1:85"

I have added what I consider the bare minimum of safety checks, such as a limit on the number of symbols it will create. You also lose help from the compiler which can’t warn you about unused variables and you may get warned that some of your variables have been assumed special (at least on LispWorks). Overall, I think its a bit of hack and not something I would put in production code.

It is also worth noting the following loop achieves the same thing, without the hackery and is trivial to change to a vector if you’re worried about efficiency:

(let ((start 80))
  (loop for port from start to (+ start 5)
        collecting (format nil "127.0.0.1:~A" port) into ports
        finally (progn
                  (setf (first ports) "http")
                  (return (values (first ports) (sixth ports))))))

Try not to fight so much with macros. I see what you’re trying to do, but it might be better to just bind a list or array of ports, instead of binding port{1-n}.

If you agree to use an array for binding, then you can use https://github.com/tdrhq/easy-macros to write this macro without using any backquotes at all. The macro definition would look something like:

(def-easy-macro with-free-ports (start end &binding ports &fn fn)
  (unwind-protect
     (fn (... construct list or array of ports ...))
    (cleanup #| this cleanup is probably the reason you're using macros in the first place |#)))

And you’ll call it as:

(with-free-ports (1000 2000 port)
  (... do something with port ...))

If you compile code, then the compiler expands macro forms at compile-time. If you want to generate a variable number of let bindings, then the number needs to be known at compile-time.

start and end thus can’t be variables at run-time.

(let ((start 1))
  (with-free-ports start 3 port-1))

If we compile above form, then at compile-time the value of start generally is unknown.