推荐使用一个类库RealReachability，复制名字到github就能找到。

使用RealReachability的好处：
1.现在很流行的公用wifi，需要网页鉴权，鉴权之前无法上网，但本地连接已经建立；
2.存在了本地网络连接，但信号很差，实际无法连接到服务器；
3.iOS连接的路由设备本身没有连接外网。

使用前要先开启网络检测：

[GLobalRealReachability startNotifier];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(netWorkChange:) name:kRealReachabilityChangedNotification object:nil];

实现网络变化全局监听：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    [GLobalRealReachability startNotifier];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(netWorkChange:) name:kRealReachabilityChangedNotification object:nil];
    return YES;
}

- (void)netWorkChange:(NSNotification *)notification{
    MBProgressHUD *HUD = [[MBProgressHUD alloc] initWithWindow:self.window];
    HUD.mode = MBProgressHUDModeText;
    [self.window addSubview:HUD];
    HUD.removeFromSuperViewOnHide = YES;
    [HUD show:YES];
    [HUD hide:YES afterDelay:1.5f];
    
    RealReachability *reachability = (RealReachability *)notification.object;
    ReachabilityStatus status = [reachability currentReachabilityStatus];
    
    switch (status) {
        case RealStatusNotReachable:
        {
            HUD.labelText = @"无网络";
        }
            break;
        case RealStatusViaWiFi:
        {
            HUD.labelText = @"WIFI";
        }
            break;
        case RealStatusViaWWAN:
        {
            HUD.labelText = @"3G";
        }
            break;
            
        default:
            break;
    }
    
}

实现手动监听

- (void)doButtonAction{
    MBProgressHUD *HUD = [[MBProgressHUD alloc] initWithView:self.view];
    HUD.mode = MBProgressHUDModeText;
    [self.view addSubview:HUD];
    HUD.removeFromSuperViewOnHide = YES;
    [HUD show:YES];
    [HUD hide:YES afterDelay:1.5f];
    
    [GLobalRealReachability reachabilityWithBlock:^(ReachabilityStatus status) {
        switch (status) {
            case RealStatusNotReachable:
            {
                HUD.labelText = @"无网络";
            }
                break;
            case RealStatusViaWiFi:
            {
                HUD.labelText = @"WIFI";
            }
                break;
            case RealStatusViaWWAN:
            {
                HUD.labelText = @"3G";
            }
                break;
                
            default:
                break;
        }
    }];
}