A TCP connection is created between an IP address and a port number in the local end and an IP address and a port number in the remote end. The remote port number can be specified in the URL and usually helps identify which service you are targeting.
The local port number is usually randomly assigned to your TCP connection by the network stack and you normally do not have to think about it much further. However, in some circumstances you find yourself behind network equipment, firewalls or similar setups that put restrictions on what source port numbers that can be allowed to set up the outgoing connections.
For situations like this, you can specify which local ports curl should bind the connection to. You can specify a single port number to use, or a range of ports. We recommend using a range because ports are scarce resources and the exact one you want may already be in use. If you ask for a local port number (or range) that curl cannot obtain for you, it exits with a failure.
Also, on most operating systems you cannot bind to port numbers below 1024 without having a higher privilege level (root) and we generally advise against running curl as root if you can avoid it.
Ask curl to use a local port number between 4000 and 4200 when getting this HTTPS page:
curl --local-port 4000-4200 https://example.com/