![]() In order to not get in the way of services that may later want to use proprietary "registered" ports, the network stack will typically try to avoid using either well-known or registered ports for this, and instead will restrict itself to the ephemeral/dynamic ports between 4915. When a client or server process asks to listen on a port, or asks to initiate an outgoing TCP connection or UDP flow, and doesn't specify a particular port, the TCP and UDP networking code in the kernel (the "network stack") assigns them a currently unused port arbitrarily. It's considered best practice for an OS to try not to use these ports. Same with SSH (22), Telnet (23), SMTP (25), FTP (21), etc.Ä«etween 102 are a bunch ports that have been assigned as the default port for a lot of not-so-well-known, especially corporate/proprietary protocols. Random User shouldn't be able to run an HTTP server on the box and make it seem like that user's HTTP server is THE OFFICIAL HTTP server for that box by running it on port 80. This is under the assumption that your Unix box is actually a multi-user system, and J. Historically, a lot of well known protocols, such as HTTP (80), have been assigned default ports below 1024, so many OSes, including most Unixes and Unix-like OSes, don't allow you to open a listener on those ports unless you have administrator/root credentials. NAPT, PAT) gateway, and any of them can be used for a client to initiate a connection from. Any of them can be used for a service/daemon to listen on, any of them can be port-forwarded in a NAT (a.k.a. As far as the TCP and UDP protocols are concerned, ports are all the same. ![]()
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