if left on?
Let’s be clear that we’re talking about dial-up modems. By definition
broadband modems are connected all the time.
In fact, if you think about it, a dialup modem that’s always connected is
pretty much just a really slow broadband modem.
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I will clarify that by “on” you mean connected. It’s certainly ok to leave
your modem powered on at all times, whether or not you have dialed in. Dialing
in and connecting to the internet is where the issues are.
You can certainly can leave it connected all the time if your ISP is ok with
that. Many ISPs are not, and will drop the connection automatically after some
period of inactivity. There are tricks you can play, of course, but those may
well violate the terms of service with your ISP. So definitely check with them
before staying connected continuously.
If you do stay connected for long periods of time, you should start
thinking like a broadband user. That means taking all the precautions that a
broadband user would take, including a firewall of some sort. Normally those
steps aren’t as important for dialup users because the connections are
typically of a shorter duration. You’re never 100% safe, but shorter
connections mean fewer opportunities for infection and other problems.
So go ahead and connect, if you’re allowed, and simply take the extra
precautions of a firewall, and of course, up to date virus protection.