Swift Alamofire discover in LAN

Question

In my LAN, I run the following Terminal command on 6 MAC computers to host a folder in which is discover.xml:

python -m SimpleHTTPServer 17171

On my app having Alamofire installed, I run this code in order to get discover.xml content:

extension String {
    func url(file: String) -> URL {
        return URL(string: "http://\(self)/\(file)?rand=\(arc4random())")! // `rand` to make sure no cached content
    }
}

func discover(from ip: String, completion: @escaping () -> Void) {
     let url = ip.url(file: "discover.xml") // ip passed in: "192.168.1.15:17171" for ex

    var request = URLRequest(url: url)
    request.httpMethod = "GET"
    request.timeoutInterval = 6.0 // !

    let manager = Alamofire.SessionManager.default
    manager.session.configuration.timeoutIntervalForRequest = 6.0
    manager.session.configuration.timeoutIntervalForResource = 6.0

    manager.request(request).responseData { (response) in
        switch response.result {
        case .success(let d):
            // do my stuff on `multiX`

        case .failure(let error):
            print("failed with IP: \(ip) => \(error)")
        }
    }        
} 

Below is how I discover:

private func discover(on subnet: String) { // subnet: 192.168.1
    let group = DispatchGroup()

    for i in 1..<256 {
        let ip = subnet + ".\(i):17171"

        group.enter()
        discover(from: ip) {
            group.leave()
        }
    }

    group.notify(queue: DispatchQueue.main) { [weak self] in
        guard let strongSelf = self else { return }

        print("all done: \(strongSelf.multiX.count)") // just can discover 2, or 3, or 4 !!
        strongSelf.tableView.reloadData()
    }
}   

Why most of the time, I cannot get 6 discover.xml from the 6 MAC computers? (I can only get 2 or 3, or 4). The result that I get is not persistent. Say I have 6 MAC computers running python -m ... named: A, B, C, D, E, F. Sometimes I can only get: A, B, D, sometimes C, F, ...

Requesting the remaining computers which host discover.xml always have error failed with IP: x.x.x.x:17171 => Error Domain=NSURLErrorDomain Code=-1001 "The request timed out." (I'm sure those missing computers have acquired the IP successfully)

Is it ok to discover the whole subnet using my method above? (i.e. using for loop to iterate each ip). Is 6 seconds a reasonable timeout?

EDIT:

I have tried to allow all in ATS, but things are not better

What actually blocks me from requesting discover.xml on the MAC computers?

Thanks,