diff --git a/pkgs/htdp-pkgs/htdp-lib/2htdp/private/clauses-spec-aux.rkt b/pkgs/htdp-pkgs/htdp-lib/2htdp/private/clauses-spec-aux.rkt
index 1aaa205d61..c14de56e4b 100644
--- a/pkgs/htdp-pkgs/htdp-lib/2htdp/private/clauses-spec-aux.rkt
+++ b/pkgs/htdp-pkgs/htdp-lib/2htdp/private/clauses-spec-aux.rkt
@@ -3,7 +3,7 @@
 ;; ---------------------------------------------------------------------------------------------------
 ;; provides constants and functions for specifying the shape of clauses in big-bang and universe 
 
-(provide nat> nat? proc> bool> num> ip> string> symbol> string-or-symbol> any> K False True)
+(provide port> nat> nat? proc> bool> num> ip> string> symbol> string-or-symbol> any> K False True)
 
 (require htdp/error "check-aux.rkt")
 
@@ -11,6 +11,10 @@
 (define (False w) #f)
 (define (True w) #t)
 
+;; Symbol X -> X : nat?
+(define (port> tag x)
+  (nat> tag x "port"))
+
 ;; Symbol X -> X : boolean? 
 (define (bool> tag x)
   (check-arg tag (boolean? x) "boolean" "first" x)
diff --git a/pkgs/htdp-pkgs/htdp-lib/2htdp/private/universe.rkt b/pkgs/htdp-pkgs/htdp-lib/2htdp/private/universe.rkt
index f925678a5d..7dfe33a158 100644
--- a/pkgs/htdp-pkgs/htdp-lib/2htdp/private/universe.rkt
+++ b/pkgs/htdp-pkgs/htdp-lib/2htdp/private/universe.rkt
@@ -58,6 +58,7 @@
        universe0         ;; the initial state of the universe
        on-new            ;; Universe World -> Result
        on-msg            ;; Universe World Message -> Result
+       port              ;; Number
        ;; tick              ;; Universe -> Result
        (state #f)        ;; Boolean 
        (on-disconnect    ;; Universe World -> Result
@@ -157,7 +158,7 @@
           ;;; WHERE 
           (define tcp-listener 
             (with-handlers ((exn:fail:network? (lambda (x) (stop! x))))
-              (tcp-listen SQPORT 4 #t)))
+              (tcp-listen port 4 #t)))
           ;; (list IPort OPort) -> Void 
           (define (add-iworld in-out)
             (define in (first in-out))
diff --git a/pkgs/htdp-pkgs/htdp-lib/2htdp/private/world.rkt b/pkgs/htdp-pkgs/htdp-lib/2htdp/private/world.rkt
index 41e85acb3a..fe9985d536 100644
--- a/pkgs/htdp-pkgs/htdp-lib/2htdp/private/world.rkt
+++ b/pkgs/htdp-pkgs/htdp-lib/2htdp/private/world.rkt
@@ -59,7 +59,7 @@
     (class* object% (start-stop<%>)
       (inspect #f)
       (init-field world0)
-      (init-field name state register check-with on-key on-release on-pad on-mouse record?)
+      (init-field name state register port check-with on-key on-release on-pad on-mouse record?)
       (init on-receive on-draw stop-when)
       
       ;; -----------------------------------------------------------------------
@@ -114,7 +114,7 @@
                                (if (= n 1) 
                                    (printf FMTtry register TRIES)
                                    (begin (sleep PAUSE) (try (- n 1)))))))
-              (define-values (in out) (tcp-connect register SQPORT))
+              (define-values (in out) (tcp-connect register port))
               (tcp-register in out name)
               (printf "... successful registered and ready to receive\n")
               (set! *out* out)
diff --git a/pkgs/htdp-pkgs/htdp-lib/2htdp/universe.rkt b/pkgs/htdp-pkgs/htdp-lib/2htdp/universe.rkt
index c6a4d64472..1ad75fcb4e 100644
--- a/pkgs/htdp-pkgs/htdp-lib/2htdp/universe.rkt
+++ b/pkgs/htdp-pkgs/htdp-lib/2htdp/universe.rkt
@@ -27,7 +27,7 @@
          ;; 
          (only-in "private/launch-many-worlds.rkt" launch-many-worlds launch-many-worlds/proc)
          (only-in "private/stop.rkt" make-stop-the-world)
-         (only-in "private/check-aux.rkt" sexp?)
+         (only-in "private/check-aux.rkt" sexp? SQPORT)
          (only-in "private/pad.rkt" pad-event? pad=?)
          htdp/error
          (rename-in lang/prim (first-order->higher-order f2h)))
@@ -74,7 +74,10 @@
   [state DEFAULT #'#f (expr-with-check any> "expected a boolean or a string")]
   ;; Any -> Boolean 
   ;; -- check-with: all states should specify this predicate 
-  [check-with DEFAULT #'True (function-with-arity 1)])
+  [check-with DEFAULT #'True (function-with-arity 1)]
+  ;; Natural
+  ;; -- port: specify the port to use
+  [port DEFAULT #'SQPORT (expr-with-check port> "expected a port number")])
 
 ;  (create-world world0)
 (define-keywords WldSpec AllSpec create-world