RCViewController.m

static void reachabilityCallback(SCNetworkReachabilityRef reachabilityRef, SCNetworkReachabilityFlags flags, void* info)
{
    RCViewController *viewController = (__bridge RCViewController*)info; //Cast context object.
    [viewController showReachabilityFlags:flags]; //Show.
    
    //Tear down reachability.
    SCNetworkReachabilityUnscheduleFromRunLoop(reachabilityRef, CFRunLoopGetMain(), kCFRunLoopDefaultMode);
    CFRelease(reachabilityRef);
}

-(void)reach:(NSString*) hostName
{
    //Create with context.
    SCNetworkReachabilityRef reachabilityRef = SCNetworkReachabilityCreateWithName(NULL, [hostName UTF8String]);
    SCNetworkReachabilityContext context = {0, (__bridge void*)self, nil, nil, nil};
    
    if (NO) //The synchronous way, works well with IP addresses as hostname.
    {
        //Get flags.
        SCNetworkReachabilityFlags flags;
        SCNetworkReachabilityGetFlags(reachabilityRef, &flags);
        
        [self showReachabilityFlags:flags]; //Show.
    }
    
    else //The asynchronous way, NOT WORKS (!!!) with IP addresses as hostname.
    {
        //Set callback, then register.
        SCNetworkReachabilitySetCallback(reachabilityRef, reachabilityCallback, &context);
        SCNetworkReachabilityScheduleWithRunLoop(reachabilityRef, CFRunLoopGetMain(), kCFRunLoopDefaultMode);
    }
}