diff --git a/collects/unstable/contract.rkt b/collects/unstable/contract.rkt
index 2b8b9af8b7..91cdf8c04c 100644
--- a/collects/unstable/contract.rkt
+++ b/collects/unstable/contract.rkt
@@ -7,6 +7,7 @@
 ;;   from this (see `string->path-element') .
 
 (define port-number? (between/c 1 65535))
+(define tcp-listen-port? (between/c 0 65535))
 
 (define non-empty-string/c
   (and/c string?
@@ -364,6 +365,7 @@
  [non-empty-string/c contract?]
  [path-element? contract?]
  [port-number? contract?]
+ [tcp-listen-port? contract?]
 
  [non-empty-string? predicate/c]
  [non-empty-bytes? predicate/c]
diff --git a/collects/unstable/scribblings/contract.scrbl b/collects/unstable/scribblings/contract.scrbl
index cf21eca5b5..41fa0362a5 100644
--- a/collects/unstable/scribblings/contract.scrbl
+++ b/collects/unstable/scribblings/contract.scrbl
@@ -15,6 +15,10 @@ Contract for non-empty strings.
 Equivalent to @racket[(between/c 1 65535)].
 }
 
+@defthing[tcp-listen-port? contract?]{
+Equivalent to @racket[(between/c 0 65535)].
+}
+
 @defthing[path-element? contract?]{
 Equivalent to @racket[(or/c path-string? (symbols 'up 'same))].
 }
diff --git a/collects/web-server/http/request.rkt b/collects/web-server/http/request.rkt
index 9f65d7b1a6..4fdbf986a6 100644
--- a/collects/web-server/http/request.rkt
+++ b/collects/web-server/http/request.rkt
@@ -1,6 +1,7 @@
 #lang racket
 (require net/url
          net/uri-codec
+         unstable/contract
          web-server/private/util
          web-server/private/connection-manager
          web-server/http/request-structs)
@@ -8,7 +9,7 @@
 (provide/contract
  [rename ext:read-request read-request
          (connection? 
-          port-number?
+          tcp-listen-port?
           (input-port? . -> . (values string? string?))
           . -> .
           (values request? boolean?))])
diff --git a/collects/web-server/insta/insta.rkt b/collects/web-server/insta/insta.rkt
index bf7d28097c..c145d8320b 100644
--- a/collects/web-server/insta/insta.rkt
+++ b/collects/web-server/insta/insta.rkt
@@ -63,5 +63,6 @@
                                    'you 'web-server
                                    "start"
                                    #f)
+                         #:port 0
                          #:extra-files-paths (if extra-files-path (list extra-files-path) empty)
                          #:launch-browser? launch-browser?))))]))
diff --git a/collects/web-server/private/dispatch-server-sig.rkt b/collects/web-server/private/dispatch-server-sig.rkt
index e7a280700e..06847143ac 100644
--- a/collects/web-server/private/dispatch-server-sig.rkt
+++ b/collects/web-server/private/dispatch-server-sig.rkt
@@ -1,6 +1,7 @@
 #lang racket
 (require racket/async-channel
          web-server/private/util
+         unstable/contract
          web-server/private/connection-manager)
 
 (define-signature dispatch-server^
@@ -10,13 +11,13 @@
 
 (define-signature dispatch-server-config^
   ((contracted
-    [port port-number?]
+    [port tcp-listen-port?]
     [listen-ip (or/c string? false/c)]
     [max-waiting integer?]
     [initial-connection-timeout integer?]
     [read-request
      (connection? 
-      port-number?
+      tcp-listen-port?
       (input-port? . -> . (values string? string?))
       . -> .
       (values any/c boolean?))]
diff --git a/collects/web-server/private/dispatch-server-unit.rkt b/collects/web-server/private/dispatch-server-unit.rkt
index 5d0c7fd2b3..e29e448691 100644
--- a/collects/web-server/private/dispatch-server-unit.rkt
+++ b/collects/web-server/private/dispatch-server-unit.rkt
@@ -24,21 +24,22 @@
     (start-connection-manager)
     (thread
      (lambda ()
-       (run-server config:port 
+       (run-server 1 ; This is the port argument, but because we specialize listen, it is ignored. 
                    handle-connection
                    #f
                    (lambda (exn)
                      ((error-display-handler) 
                       (format "Connection error: ~a" (exn-message exn))
                       exn))
-                   (lambda (p mw re)
+                   (lambda (_ mw re)
                      (with-handlers ([exn? 
                                       (λ (x)
                                         (async-channel-put* confirmation-channel x)
                                         (raise x))])
-                       (begin0
-                         (tcp-listen p config:max-waiting #t config:listen-ip)
-                         (async-channel-put* confirmation-channel #f))))
+                       (define listener (tcp-listen config:port config:max-waiting #t config:listen-ip))
+                       (let-values ([(local-addr local-port end-addr end-port) (tcp-addresses listener #t)])
+                         (async-channel-put* confirmation-channel local-port))
+                       listener))
                    tcp-close
                    tcp-accept
                    tcp-accept/enable-break))))
diff --git a/collects/web-server/scribblings/dispatch-server.scrbl b/collects/web-server/scribblings/dispatch-server.scrbl
index 35dfe2befa..16ffaa7c1c 100644
--- a/collects/web-server/scribblings/dispatch-server.scrbl
+++ b/collects/web-server/scribblings/dispatch-server.scrbl
@@ -7,6 +7,7 @@
                      web-server/private/util
                      web-server/private/connection-manager
                      net/tcp-sig
+                     unstable/contract
                      racket/async-channel
                      racket/tcp
                      web-server/web-server-sig))
@@ -26,7 +27,7 @@ The @racket[dispatch-server^] signature is an alias for
 @racket[web-server^].
 
  @defproc[(serve) (->* () (#:confirmation-channel (or/c false/c async-channel?)) (-> void))]{
-  Runs the server---the confirmation channel will be send an exception if one occurs starting the server or @racket[#f] if the is none---and returns a procedure that shuts down the server.
+  Runs the server---the confirmation channel will be send an exception if one occurs starting the server or the port number if there is none---and returns a procedure that shuts down the server.
  }
 
  @defproc[(serve-ports [ip input-port?]
@@ -39,12 +40,12 @@ The @racket[dispatch-server^] signature is an alias for
 
 @defsignature[dispatch-server-config^ ()]{
 
- @defthing[port port-number?]{Specifies the port to serve on.}
+ @defthing[port tcp-listen-port?]{Specifies the port to serve on.}
  @defthing[listen-ip (or/c string? false/c)]{Passed to @racket[tcp-listen].}
  @defthing[max-waiting integer?]{Passed to @racket[tcp-accept].}
  @defthing[initial-connection-timeout integer?]{Specifies the initial timeout given to a connection.}
  @defproc[(read-request [c connection?]
-                        [p port-number?]
+                        [p tcp-listen-port?]
                         [port-addresses 
                          (input-port? . -> . (values string? string?))])
           (values any/c boolean?)]{
diff --git a/collects/web-server/scribblings/launch.scrbl b/collects/web-server/scribblings/launch.scrbl
index aa23b73f5e..fe9e406aa5 100644
--- a/collects/web-server/scribblings/launch.scrbl
+++ b/collects/web-server/scribblings/launch.scrbl
@@ -11,6 +11,7 @@
                      web-server/private/dispatch-server-sig
                      web-server/dispatchers/dispatch
                      racket/async-channel
+                     unstable/contract
                      web-server/configuration/configuration-table)
           (prefix-in raw: (for-label net/tcp-unit))
           (prefix-in files: (for-label web-server/dispatchers/dispatch-files)))
@@ -22,7 +23,7 @@ This module provides functions for launching dispatching servers.
 @defproc[(serve [#:dispatch dispatch dispatcher/c]
                 [#:confirmation-channel confirmation-channel (or/c false/c async-channel?) #f]
                 [#:tcp@ tcp@ (unit/c (import) (export tcp^)) raw:tcp@]
-                [#:port port integer? 80]
+                [#:port port tcp-listen-port? 80]
                 [#:listen-ip listen-ip (or/c string? false/c) #f]
                 [#:max-waiting max-waiting integer? 40]
                 [#:initial-connection-timeout initial-connection-timeout integer? 60])
@@ -51,7 +52,7 @@ from a given path:
 @defproc[(serve/ports [#:dispatch dispatch dispatcher/c]
                       [#:confirmation-channel confirmation-channel (or/c false/c async-channel?) #f]
                       [#:tcp@ tcp@ (unit/c (import) (export tcp^)) raw:tcp@]
-                      [#:ports ports (listof integer?) (list 80)]
+                      [#:ports ports (listof tcp-listen-port?) (list 80)]
                       [#:listen-ip listen-ip (or/c string? false/c) #f]
                       [#:max-waiting max-waiting integer? 40]
                       [#:initial-connection-timeout initial-connection-timeout integer? 60])
@@ -63,7 +64,7 @@ from a given path:
 @defproc[(serve/ips+ports [#:dispatch dispatch dispatcher/c]
                           [#:confirmation-channel confirmation-channel (or/c false/c async-channel?) #f]
                           [#:tcp@ tcp@ (unit/c (import) (export tcp^)) raw:tcp@]
-                          [#:ips+ports ips+ports (listof (cons/c (or/c string? false/c) (listof integer?))) (list (cons #f (list 80)))]
+                          [#:ips+ports ips+ports (listof (cons/c (or/c string? false/c) (listof tcp-listen-port?))) (list (cons #f (list 80)))]
                           [#:max-waiting max-waiting integer? 40]
                           [#:initial-connection-timeout initial-connection-timeout integer? 60])
          (-> void)]{
diff --git a/collects/web-server/scribblings/servlet-env.scrbl b/collects/web-server/scribblings/servlet-env.scrbl
index 7ebcb3a8af..73289454c8 100644
--- a/collects/web-server/scribblings/servlet-env.scrbl
+++ b/collects/web-server/scribblings/servlet-env.scrbl
@@ -128,7 +128,7 @@ and if @racket[serve/servlet] is run in another module.
                         [#:quit? quit? boolean? (not command-line?)]
                         [#:banner? banner? boolean? (not command-line?)]
                         [#:listen-ip listen-ip (or/c false/c string?) "127.0.0.1"]
-                        [#:port port number? 8000]
+                        [#:port port tcp-listen-port? 8000]
                         [#:servlet-path servlet-path string?
                                         "/servlets/standalone.rkt"]
                         [#:servlet-regexp servlet-regexp regexp?
diff --git a/collects/web-server/servlet-dispatch.rkt b/collects/web-server/servlet-dispatch.rkt
index d41553313b..187f9d57a7 100644
--- a/collects/web-server/servlet-dispatch.rkt
+++ b/collects/web-server/servlet-dispatch.rkt
@@ -9,6 +9,7 @@
          racket/serialize
          net/tcp-unit
          net/tcp-sig
+         unstable/contract
          net/ssl-tcp-unit)
 (require web-server/web-server
          web-server/managers/lru
@@ -37,7 +38,7 @@
                      (#:launch-path (or/c false/c string?)
                                     #:banner? boolean?
                                     #:listen-ip (or/c false/c string?)
-                                    #:port number?
+                                    #:port tcp-listen-port?
                                     #:ssl-cert (or/c false/c path-string?)
                                     #:ssl-key (or/c false/c path-string?))
                      . ->* .
@@ -93,24 +94,19 @@
          #:listen-ip
          [listen-ip "127.0.0.1"]
          #:port
-         [port 8000]
+         [port-arg 8000]
          #:ssl-cert
          [ssl-cert #f]
          #:ssl-key
          [ssl-key #f])
   (define ssl? (and ssl-cert ssl-key))
-  (define server-url
-    (string-append (if ssl? "https" "http")
-                   "://localhost"
-                   (if (and (not ssl?) (= port 80))
-                       "" (format ":~a" port))))
   (define sema (make-semaphore 0))
   (define confirm-ch (make-async-channel 1))
   (define shutdown-server
     (serve #:confirmation-channel confirm-ch
            #:dispatch (dispatcher sema)
            #:listen-ip listen-ip
-           #:port port
+           #:port port-arg
            #:tcp@ (if ssl?
                       (let ()
                         (define-unit-binding ssl-tcp@
@@ -121,15 +117,20 @@
                         ssl-tcp@)
                       tcp@)))
   (define serve-res (async-channel-get confirm-ch))
-  (if serve-res
+  (if (exn? serve-res)
       (begin
         (when banner? (eprintf "There was an error starting the Web server.\n"))
         (match serve-res
-          [(and (? exn?) (app exn-message (regexp "tcp-listen: listen on .+ failed \\(Address already in use; errno=.+\\)" (list _))))
-           (when banner? (eprintf "\tThe TCP port (~a) is already in use.\n" port))]
+          [(app exn-message (regexp "tcp-listen: listen on .+ failed \\(Address already in use; errno=.+\\)" (list _)))
+           (when banner? (eprintf "\tThe TCP port (~a) is already in use.\n" port-arg))]
           [_
            (void)]))
-      (begin
+      (local [(define port serve-res)
+              (define server-url
+                (string-append (if ssl? "https" "http")
+                               "://localhost"
+                               (if (and (not ssl?) (= port 80))
+                                   "" (format ":~a" port))))]
         (when launch-path
           ((send-url) (string-append server-url launch-path) #t))
         (when banner?
diff --git a/collects/web-server/servlet-env.rkt b/collects/web-server/servlet-env.rkt
index ddb6bc2a62..b0d4535f7c 100644
--- a/collects/web-server/servlet-env.rkt
+++ b/collects/web-server/servlet-env.rkt
@@ -14,6 +14,7 @@
          web-server/private/mime-types
          web-server/servlet/setup
          web-server/servlet-dispatch
+         unstable/contract
          (prefix-in lift: web-server/dispatchers/dispatch-lift)
          (prefix-in fsmap: web-server/dispatchers/filesystem-map)
          (prefix-in sequencer: web-server/dispatchers/dispatch-sequencer)
@@ -43,7 +44,7 @@
                   #:quit? boolean?
                   #:banner? boolean?
                   #:listen-ip (or/c false/c string?)
-                  #:port number?
+                  #:port tcp-listen-port?
                   #:ssl? boolean?
                   #:ssl-cert (or/c false/c path-string?)
                   #:ssl-key (or/c false/c path-string?)
diff --git a/collects/web-server/web-server.rkt b/collects/web-server/web-server.rkt
index fe18583cf3..82f600adf9 100644
--- a/collects/web-server/web-server.rkt
+++ b/collects/web-server/web-server.rkt
@@ -5,6 +5,7 @@
          racket/unit
          racket/async-channel
          racket/contract
+         unstable/contract
          web-server/dispatchers/dispatch
          web-server/private/dispatch-server-sig
          web-server/private/dispatch-server-unit
@@ -17,7 +18,7 @@
   (->* (#:dispatch dispatcher/c)
        (#:confirmation-channel (or/c false/c async-channel?)
         #:tcp@ (unit/c (import) (export tcp^))
-        #:port number?
+        #:port tcp-listen-port?
         #:listen-ip (or/c false/c string?)
         #:max-waiting number?
         #:initial-connection-timeout number?)
@@ -26,7 +27,7 @@
   (->* (#:dispatch dispatcher/c)
        (#:confirmation-channel (or/c false/c async-channel?)
         #:tcp@ (unit/c (import) (export tcp^))
-        #:ports (listof number?)
+        #:ports (listof tcp-listen-port?)
         #:listen-ip (or/c false/c string?)
         #:max-waiting number?
         #:initial-connection-timeout number?)
@@ -35,7 +36,7 @@
   (->* (#:dispatch dispatcher/c)
        (#:confirmation-channel (or/c false/c async-channel?)
         #:tcp@ (unit/c (import) (export tcp^))
-        #:ips+ports (listof (cons/c (or/c false/c string?) (listof number?)))
+        #:ips+ports (listof (cons/c (or/c false/c string?) (listof tcp-listen-port?)))
         #:max-waiting number?
         #:initial-connection-timeout number?)
        (-> void))]