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- [Instructor] Layer 4 of the OSI model, transport,

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uses a 16-bit number, known as a port,

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to facilitate endpoint communication.

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TCP and UDP both open lines of communication using ports.

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Applications will open up a port

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and wait for an incoming connection.

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This is also referred to as listening.

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For well-known applications,

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these will be well-known port numbers

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like port 80 for HTTP and 443 for HTTPS.

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Once the client application

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connects to the well-known ports,

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the server will sometimes shift the client

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to a different port to complete the transaction.

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Well-known ports are generally less than 1024.

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Ephemeral ports are those above 1024 and are usually used

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for short-term communications by clients.

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Whenever a packet is created and sent,

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the header in that packet will specify

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both a source and destination port.

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Since the port number is 16-bit,

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it can be zero to 65,535.

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TCP can't use port zero as it's reserved.

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With UDP, the source port number is optional

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and a value of zero means no port is used.

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An internet socket is created using a transport protocol,

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a port number, and an IP address.

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This process is known as binding.

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Per host, only a single binding is allowed

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on a port and IP combination.

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Otherwise, conflicts can occur.

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When troubleshooting server software that won't start,

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an admin may run into the port already in use issue.

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This generally stems from another instance

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of the software already running

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or perhaps another application consuming the same port.

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It's not only end hosts and servers that use ports.

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Firewalls, routers, and switches also examine port numbers.

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Firewalls use port numbers to allow connections

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through to various services.

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They also use these port numbers

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to recognize what types of traffic to expect

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and can do additional packet scrutiny

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to ensure they conform.

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Routers and switches can also use port numbers

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for quality of service.

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They can use these port designations

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to identify certain types of traffic

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and elevate their priority as they move through the network.

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This can, for example, ensure that VoIP traffic

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has a higher priority than web surfing.

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Ports are integral to the network

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and server's functionality.