There is a special sort of defeat in seeing “421 Too many connections from this IP” after thirty minutes of uploading. It means the server has limited how many connections one address may hold at once, and you have used them all.

You probably did not do it deliberately. FTP clients open a new connection for each transfer, and failed transfers can leave half-open connections on the server until they expire. Retries stack up. Soon the server counts twenty connections and refuses the lot.

A shared network makes it worse. If a whole classroom sits behind one public IP address, everyone shares the same limit, and your neighbour’s enthusiastic upload is now your problem.

The cure is gentle. Set your client to one simultaneous connection and one simultaneous transfer. Upload one archive instead of thousands of files. Wait ten minutes or so for old sessions to expire, then try again. If it persists, ask your host to clear your sessions or check whether their firewall has blocked you.

Resist the urge to click Reconnect fifty times. The server counts that as enthusiasm, not progress.

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