diff --git a/test/net/http/test_http.rb b/test/net/http/test_http.rb
index 660c0ff..2d81668 100644
--- a/test/net/http/test_http.rb
+++ b/test/net/http/test_http.rb
@@ -426,7 +426,7 @@ end
 class TestNetHTTP_v1_2 < Test::Unit::TestCase
   CONFIG = {
     'host' => '127.0.0.1',
-    'port' => 10081,
+    'port' => 0,
     'proxy_host' => nil,
     'proxy_port' => nil,
   }
@@ -444,7 +444,7 @@ end
 class TestNetHTTP_v1_2_chunked < Test::Unit::TestCase
   CONFIG = {
     'host' => '127.0.0.1',
-    'port' => 10081,
+    'port' => 0,
     'proxy_host' => nil,
     'proxy_port' => nil,
     'chunked' => true,
@@ -476,7 +476,7 @@ end
 class TestNetHTTPContinue < Test::Unit::TestCase
   CONFIG = {
     'host' => '127.0.0.1',
-    'port' => 10081,
+    'port' => 0,
     'proxy_host' => nil,
     'proxy_port' => nil,
     'chunked' => true,
diff --git a/test/net/http/test_https.rb b/test/net/http/test_https.rb
index 983ba7f..2f2c699 100644
--- a/test/net/http/test_https.rb
+++ b/test/net/http/test_https.rb
@@ -24,7 +24,7 @@ class TestNetHTTPS < Test::Unit::TestCase
 
   CONFIG = {
     'host' => '127.0.0.1',
-    'port' => 10082, # different from test_http.rb
+    'port' => 0,
     'proxy_host' => nil,
     'proxy_port' => nil,
     'ssl_enable' => true,
diff --git a/test/net/http/utils.rb b/test/net/http/utils.rb
index 50f616f..db01b18 100644
--- a/test/net/http/utils.rb
+++ b/test/net/http/utils.rb
@@ -19,7 +19,8 @@ module TestNetHTTPUtils
   end
 
   def config(key)
-    self.class::CONFIG[key]
+    @config ||= self.class::CONFIG
+    @config[key]
   end
 
   def logfile
@@ -42,6 +43,7 @@ module TestNetHTTPUtils
   end
 
   def spawn_server
+    @config = self.class::CONFIG
     server_config = {
       :BindAddress => config('host'),
       :Port => config('port'),
@@ -61,6 +63,7 @@ module TestNetHTTPUtils
     @server = WEBrick::HTTPServer.new(server_config)
     @server.mount('/', Servlet, config('chunked'))
     @server.start
+    @config['port'] = @server[:Port] if @config['port'] == 0
     n_try_max = 5
     begin
       TCPSocket.open(config('host'), config('port')).close