My father-in-law's ISP warned him in the strictest possible terms to physically turn off his rented Nokia Siemens C2110 PPPoE broadband router when not in use. Everyone I know leaves their broadband router on all the time, and I've never heard of any negative impacts.
Is leaving a consumer broadband router running 24/7 really problematic? Is the ISP in question being ridiculously paranoid?
The security point of view is that if a hacker can find a way to hack into your router, then it can be done within minutes. Turning the router off at night doesn't make a difference.
The only reason why the ISP advises the customers to turn off the routers is that this ISP is running at their peak capability and they want to have as few customers on-line at any given time as possible. They are running out of IP addresses to assign to PPPoE nodes or something like that.
Unless he's really serious about saving energy and is willing to potentially reduce the life of the unit and make using/accessing his computer more of a hassle, I see no compelling reason to regularly turn off a broadband modem or a router (this unit combines both).
Since this is the only modem/router I know of that actually has a power switch, who knows... So have him ask why he should turn it off.
If their response is this is a "security measure", it's a really bad one.
It's not a problem to leave it on 24/7.
Security - If you're vulnerable, you're going to be hacked within minutes of connection. The only plus side to having it off is that if you ARE hacked and turned into a zombie, it's that much less time that your machines are available to perform DDOS / act as spambots. Of course, if you shut off your computers, then it's not so much an issue. If it's a wireless router, it's that much less time that you're polluting the already very crowded WiFi portion of the spectrum. Does offer less opportunities for child porn/warez surfers to hijack your wireless and get you in trouble I suppose.
Power saving - The savings would be very small, but do this for enough devices in the house and it starts to add up. Could contribute to earlier device failure, but it'll still almost certainly be obsolete before that happens.
I don't mind having to switch it on to use it since I have my whole PC setup on a single power strip (effectively making a single power on button for everything). I'll take the power savings since it's easy enough to do in my case. Probably wouldn't work out as well for other people though (especially those with families).
I leave my router on 24/7, electronics wise they are designed to be on 24/7 and as long as you have Password protected the Network and changed the default administrator login on the box there's not much more you can do securitywise.
It would be interesting to know what their reasons are for asking him to turn it off when not in use. In England BT Homehub routers come configured with a chargable connection called BT Openzone that people can pay to use. This is configured on all Homehub 2.0's and you have to disable it, they try and monitize the fact that their Homehubs are left on all the time.
