Apple's Reachability class periodically checks the network status and alerts observers to changes.

Reachability *internetReachability = [Reachability reachabilityForInternetConnection];
[internetReachability startNotifier];

Reachability uses NSNotification messages to alert observers when the network state has changed. Your class will need to become an observer.

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(reachabilityChanged:) name:kReachabilityChangedNotification object:nil];

Elsewhere in your class, implement method signature

- (void) reachabilityChanged:(NSNotification *)note {
    //code which reacts to network changes
}

- (void)reachabilityChanged:(NSNotification *)note {
    Reachability* reachability = [note object];
    NetworkStatus netStatus = [reachability currentReachabilityStatus];

    if (netStatus == NotReachable) {
        NSLog(@"Network unavailable");
    }
}

- (void)reachabilityChanged:(NSNotification *)note {
    Reachability* reachability = [note object];
    NetworkStatus netStatus = [reachability currentReachabilityStatus];

    switch (netStatus) {
        case NotReachable:
            NSLog(@"Network unavailable");
            break;
        case ReachableViaWWAN:
            NSLog(@"Network is cellular");
            break;
        case ReachableViaWiFi:
            NSLog(@"Network is WIFI");
            break;
    }
}

Swift

import SystemConfiguration

/// Class helps to code reuse in handling internet network connections.
class NetworkHelper {

    /**
     Verify if the device is connected to internet network.
     - returns:          true if is connected to any internet network, false if is not
     connected to any internet network.
     */
   class func isConnectedToNetwork() -> Bool {
       var zeroAddress = sockaddr_in()
    
       zeroAddress.sin_len = UInt8(sizeofValue(zeroAddress))
       zeroAddress.sin_family = sa_family_t(AF_INET)
    
       let defaultRouteReachability = withUnsafePointer(&zeroAddress) {
           SCNetworkReachabilityCreateWithAddress(nil, UnsafePointer($0))
       }
    
       var flags = SCNetworkReachabilityFlags()
    
       if !SCNetworkReachabilityGetFlags(defaultRouteReachability!, &flags) {
           return false
       }
    
       let isReachable = (flags.rawValue & UInt32(kSCNetworkFlagsReachable)) != 0
       let needsConnection = (flags.rawValue & UInt32(kSCNetworkFlagsConnectionRequired)) != 0
    
      return (isReachable && !needsConnection)
   }
}



if NetworkHelper.isConnectedToNetwork() {
    // Is connected to network
}

Objective-C:

we can check network connectivity within few lines of code as:

-(BOOL)isConntectedToNetwork
{
    Reachability *networkReachability = [Reachability reachabilityForInternetConnection];
    NetworkStatus networkStatus = [networkReachability currentReachabilityStatus];
    if (networkStatus == NotReachable)
    {
        NSLog(@"There IS NO internet connection");
        return false;
    } else
    {
        NSLog(@"There IS internet connection");
        return true;
    }
}