What does it mean when my ADSL light flashes late at night?

Q  I have a D-Link modem/router that’s connected to two computers (both Windows 7) and, obviously, to my broadband internet connection.

The router has three lights, labelled Power, Status and ADSL. Power is on all the time, Status flashes on and off so long as everything is working well, and ADSL usually remains illuminated continuously.

However, I’ve noticed that late in the evening, this ADSL light flashes on and off rapidly for a short while, even when both computers are switched off. The router handbook says that when this light flashes it signifies a WAN session.

What is a WAN session? Is it possible that my router is communicating with someone even if my computers are switched off? And if so, with whom and why?
Laurence Rostron

A  To answer your question, we first need to explain the meaning of the acronym ‘WAN’. It stands for ‘wide area network’ and, while it can be used in different contexts, here it really just means ‘the internet’: your router is connecting to the wide area network that is the internet.

It does this via a server computer operated by your internet service provider – and the relevance of this point will become clear in just a moment.

Now, while you didn’t tell us the precise model of router, the phrase ‘WAN session’ does seem to have been coined by D-Link (ie, it appears in D-Link manuals but few other places). As such, we only have D-Link’s guidance to go on and as you’ve already discovered, it’s hardly very clear.

Fortunately, though, we have a good idea of what’s going on here. Essentially, the ADSL light is used to indicate when a successful ADSL (ie internet) connection has been negotiated.

During this electronic negotiation – when your router is communicating with the ISP’s server – the light will flash. You can check this by switching off the router at the mains, waiting a few seconds and then reconnecting the power. Once the negotiation is complete, the light stays illuminated.

So, why does the light start flashing late at night? The reason relates to the way many ISPs handle their customers’ individual connections to the internet (or WAN, remember). Every device connected to the internet must be assigned something called an ‘IP address’ – essentially a unique number that identifies the gadget.

In this instance, it is your router that is assigned this IP address (and the router then assigns similar but local IP addresses to each of your two computers, but that’s a digression from our explanation).

It is possible to issue static IP addresses (ie an IP address that never changes) but for a variety of technical and logistical reasons, in many cases these days ISPs assign ‘dynamic’ IP addresses on a fixed ‘lease’. Typically, the lease will be for 24 hours, at which point the IP address expires and the device using it must electronically apply for a lease on a new IP address.

All this takes just a few moments and is essentially transparent to the user, but we believe that this is why your router’s ADSL light flashes each night.

In short, around the same time each night, the ISP expires the lease on the IP address assigned to the router, so the router then goes through the motions of securing a new one to keep the internet connection alive. D-Link appears to call this process a ‘WAN session’ and the router indicates this behaviour by flashing its ADSL light.