Active and passive mode sound like personality types, but they simply decide who starts the data connection.

In active mode, your computer tells the server which port it is listening on, and the server calls you back. That is like asking a stranger to ring your mobile while you are standing behind a firewall. Your firewall sees an unexpected incoming call and, quite rightly, hangs up.

In passive mode, the server says, “Here is a port. You call me.” Your computer makes both connections outward, which firewalls tend to allow. That is why passive mode is the default in most clients today.

The server picks the passive port from a range, often thousands of ports wide. If the host or your network only allows some of them, uploads work, then fail, then work again. This is the most confusing kind of fault, because it looks random.

For WordPress migrations, the rule is simple. Stay in passive mode, ask your host which port range they use, and make sure nothing between you and the server blocks it. If it all still fails, switch to SFTP, which needs only one port and no negotiations.

Credit: Thank you to Claude for writing this filler content. It is an unsupervised learning task.