# Basic of Layer 4

Provides transparent transfer of data between end hosts.

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Provides (or doesn't provide) various services to applications:

* reliable data transfer
* error recovery
* data sequencing
* flow control

Provides Layer 4 addressing (**port** numbers).

* Identify the Application Layer protocol.
* Provides session multiplexing.

{% hint style="warning" %}
In this context, port, **does not** refer to the physical interfaces/ports on network devices.
{% endhint %}

* The following ranges have been designated by IANA (Internet Assigned Numbers Authority)
  * **Well-known** port numbers: 0 -1023
  * **Registered** port numbers: 1024 - 49151
  * **Ephemeral/private/dynamic** port numbers: 49152 - 65535