retrofit2でAPIをリクエストした際にFailed to connectのエラーで失敗する

テスト用のサーバに127.0.0.1:3000/でアクセスしようとしましたがタイトルのエラーで接続に失敗します。
考えられる原因は何でしょうか？

エラーの内容は以下です

D/RETROFIT_TEST: onFailure
                                                                                         java.net.ConnectException: Failed to connect to /127.0.0.1:3000
                                                                                             at okhttp3.internal.io.RealConnection.connectSocket(RealConnection.java:187)
                                                                                             at okhttp3.internal.io.RealConnection.buildConnection(RealConnection.java:170)
                                                                                             at okhttp3.internal.io.RealConnection.connect(RealConnection.java:111)
                                                                                             at okhttp3.internal.http.StreamAllocation.findConnection(StreamAllocation.java:187)
                                                                                             at okhttp3.internal.http.StreamAllocation.findHealthyConnection(StreamAllocation.java:123)
                                                                                             at okhttp3.internal.http.StreamAllocation.newStream(StreamAllocation.java:93)
                                                                                             at okhttp3.internal.http.HttpEngine.connect(HttpEngine.java:296)
                                                                                             at okhttp3.internal.http.HttpEngine.sendRequest(HttpEngine.java:248)
                                                                                             at okhttp3.RealCall.getResponse(RealCall.java:243)
                                                                                             at okhttp3.RealCall$ApplicationInterceptorChain.proceed(RealCall.java:201)
                                                                                             at okhttp3.logging.HttpLoggingInterceptor.intercept(HttpLoggingInterceptor.java:212)
                                                                                             at okhttp3.RealCall$ApplicationInterceptorChain.proceed(RealCall.java:190)
                                                                                             at com.example.yaritakouhei.niftyinterntest.MainActivity$1.intercept(MainActivity.java:46)
                                                                                             at okhttp3.RealCall$ApplicationInterceptorChain.proceed(RealCall.java:190)
                                                                                             at okhttp3.RealCall.getResponseWithInterceptorChain(RealCall.java:163)
                                                                                             at okhttp3.RealCall.access$100(RealCall.java:30)
                                                                                             at okhttp3.RealCall$AsyncCall.execute(RealCall.java:127)
                                                                                             at okhttp3.internal.NamedRunnable.run(NamedRunnable.java:32)
                                                                                             at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
                                                                                             at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
                                                                                             at java.lang.Thread.run(Thread.java:818)

コードは以下です。

java
1import retrofit2.Call;
2import retrofit2.http.GET;
3
4public interface ApiService {
5    @GET("/")
6    Call<RandomUserDemo> apiDemo();

java
1    @Override
2    protected void onCreate(Bundle savedInstanceState) {
3        super.onCreate(savedInstanceState);
4        setContentView(R.layout.activity_main);
5
6        //okhttpのclient作成
7        OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
8        httpClient.addInterceptor(new Interceptor() {
9            @Override
10            public Response intercept(Chain chain) throws IOException {
11                Request original = chain.request();
12
13                //header設定
14                Request request = original.newBuilder()
15                        .header("Accept", "application/json")
16                        .method(original.method(), original.body())
17                        .build();
18
19                Response response = chain.proceed(request);
20
21                return response;
22            }
23        });
24
25        //ログ出力設定
26        HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
27        logging.setLevel(HttpLoggingInterceptor.Level.BASIC);
28        httpClient.addInterceptor(logging);
29
30        //クライアント生成
31        OkHttpClient client = httpClient.build();
32        Retrofit retrofit = new Retrofit.Builder()
33                .baseUrl("http://127.0.0.1:3000/")//基本のurl設定
34                .addConverterFactory(GsonConverterFactory.create())//Gsonの使用
35                .client(client)//カスタマイズしたokhttpのクライアントの設定
36                .build();
37        //Interfaceから実装を取得
38        ApiService API = retrofit.create(ApiService.class);
39
40        //実行
41        API.apiDemo().enqueue(new Callback<RandomUserDemo>() {
42            @Override
43            public void onResponse(Call<RandomUserDemo> call, retrofit2.Response<RandomUserDemo> response) {
44                Log.d("RETROFIT_TEST","レスポンス");
45                if (response.isSuccessful()) {
46                    //通信結果をオブジェクトで受け取る
47                    RandomUserDemo demo = response.body();
48                    Log.d("RETROFIT_TEST", demo.info.seed);
49                } else {
50                    //通信が成功したが、エラーcodeが返ってきた場合はこちら
51                    Log.d("RETROFIT_TEST", "error_code" + response.code());
52                }
53            }
54
55            @Override
56            public void onFailure(Call<RandomUserDemo> call, Throwable t) {
57                //通信が失敗した場合など
58                Log.d("RETROFIT_TEST", "onFailure", t);
59            }
60        });
61
62        API.apiDemo().enqueue(new Callback<RandomUserDemo>() {
63            @Override
64            public void onResponse(Call<RandomUserDemo> call, retrofit2.Response<RandomUserDemo> response) {
65                RandomUserDemo demo = response.body();
66            }
67
68            @Override
69            public void onFailure(Call<RandomUserDemo> call, Throwable t) {
70            }
71        });
72    }