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Example uses

  • A backend webserver that is called by the world-visible webserver.
  • A static NAT table for the public to use a port number to specify which server they are connecting to.
  • A software that is developed by the company that will not be published, but used in-house.

I was just wondering if there was a standard section of port numbers that should be used. For example when it comes to DNS testing, or non-world-readable domains, there are items in RFC 2606 such as example.com, *.test, etc.

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UDP and TCP ports are assigned by IANA. The ones below 1024 are considered "the well-known ports" and shouldn't be used for anything other than their stated purpose (without good reason). The rest are pretty well fair game: Just let your users know what port to connect to, and keep some documentation for your site.

There is a range that is "unregisterable" (49152–65535) - This is the pool applications are supposed to grab from when they need to open a connection, but it's also available for you to use for whatever purposes you 'd like.

Wikipedia has a page with a prettified version of the port listing. You can also find it in /etc/services on most Unix machines.

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As far as listening ports on services are concerned, there are the IANA-registered well-known ports, most of which lie below 1024.

Apart from those, you're free to use whatever ports suit you.

A comprehensive list is here: http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers

In case you want to know, quick-and-dirty, what to use, go to 50000 and over.

Plenty of ports for everybody to go around, after all.

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Plenty of ports for everybody to go around, after all. -- Didn't they say that about IPv4 addresses? – voretaq7♦ Dec 20 '11 at 17:29
Don't say we're running out! That's just a guvmint 'spiracy! – adaptr Dec 20 '11 at 17:53
@voretaq7 if you need more ports, just use another IP address. But it's one IP address per device, not one port per device - adding more devices to the internet doesn't consume a port, each device has its own set of ports – Random832 Dec 20 '11 at 19:02
@Random832 Your logic falls apart if I need more than 64K sockets on a single address. It also falls apart in terms of assigned ports (Port numbers are a finite resource, and IANA port assignments, while non-binding, are technically global) – voretaq7♦ Dec 20 '11 at 19:10
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