Watch out for cookies, warns Rick Maybury. telegraph.co.uk - 20th October 2013
We have two computers, served through a router, in other words, a very conventional setup. This morning I was looking up double-glazing firms by typing their address into Firefox. I looked at two websites. An hour later, I was checking emails on my wife’s computer. She has a Gmail account. One of the ads in the right hand column was for one of the firms I had looked at. It was very unlikely that it was just a coincidence, as it is a fairly small firm. I find this quite scary. I do not have a Gmail account on my computer. Can adverts be connected in this way? If so, how can we stop it?
Ross Dallas, by email
It can be quite disconcerting when this sort of thing happens but it is not a mystery, or a coincidence. The reason you saw that ad on another computer is simply because they are on the same network and therefore share the same IP address. Although you didn’t use Google to get to the website concerned, it advertises with them and through a cookie on your browser it created an association between your IP address and the company. When you logged on to Gmail on your wife’s computer your common IP address was flagged up by Google, which sent a targeted ad from the company to Gmail. Google is reasonably open about how ad targeting and customisation works and there is more information, and how you can opt out of features like this at: http://goo.gl/CibpfU.