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Update overview.md

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@ -45,9 +45,17 @@ This is a living document. If you think of something that is not on this list bu
2. Create a service;
3. Then scale up the replication controller.
- Don't use `hostPort` unless it is absolutely necessary (for example: for a node daemon). It specifies the port number to expose on the host. When you bind a Pod to a `hostPort`, there are a limited number of places to schedule a pod due to port conflicts. The conflict comes from being limited to a single `hostIP:hostPort:protocol` combination. Different `hostIP:hostPort:protocol` combinations mean different requirements. For example, a pod that binds tcp 80 host port on 127.0.0.1 does not conflict with another pod that binds tcp 80 port on 127.0.0.2.
- Don't use `hostPort` unless it is absolutely necessary (for example: for a node daemon).
It specifies the port number to expose on the host.
When you bind a Pod to a `hostPort`, there are a limited number of places to schedule a pod due to port conflicts.
The conflict comes from the requirement of an unique <hostIP,hostPort,protocol> combination.
Different <hostIP,hostPort,protocol> combinations mean different requirements.
For example, a pod that binds to host port 80 on 127.0.0.1 with TCP protocol has no conflict with another Pod that binds to host port 80 on 127.0.0.2 with TCP protocol.
Special notes on hostIP and protocol, if you don't specify the hostIP and protocol explicitly, kubernetes will give you 0.0.0.0 and tcp as default hostIP and protocol. And "0.0.0.0" is a wildcard hostIP that will occupy all <_,hostPort,protocol> on the node the pod is scheduled on. Specifically, it will occupy all <IP,hostPort,protocol> for all IPs on the host.
*Special notes on hostIP and protocol*: If you don't specify the hostIP and protocol explicitly,
kubernetes will give you 0.0.0.0 and tcp as the default hostIP and protocol,
where "0.0.0.0" is a wildcard IP that will match all <*,hostPort,protocol> on the node the pod is scheduled on.
Specifically, it will match all <IP,hostPort,protocol> tuples for all IPs on the host.
If you only need access to the port for debugging purposes, you can use the [kubectl proxy and apiserver proxy](/docs/tasks/access-kubernetes-api/http-proxy-access-api/) or [kubectl port-forward](/docs/tasks/access-application-cluster/port-forward-access-application-cluster/).
You can use a [Service](/docs/concepts/services-networking/service/) object for external service access.