Merge pull request #30234 from calvin0327/improvement-install-kubeadm

Improvement for install-kubeadm

Author: k8s-ci-robot

content/en/docs/setup/production-environment/tools/kubeadm/install-kubeadm.md

@@ -69,7 +69,11 @@ For more details please see the [Network Plugin Requirements](/docs/concepts/ext
 ## Check required ports
 These
 [required ports](/docs/reference/ports-and-protocols/)
-need to be open in order for Kubernetes components to communicate with each other.
+need to be open in order for Kubernetes components to communicate with each other. You can use `telnet` to check if a port is open. for example:
+
+```shell
+telnet 127.0.0.1 6443
+```
 
 The pod network plugin you use (see below) may also require certain ports to be
 open. Since this differs with each pod network plugin, please see the