Indeed "often" does mean that, pretty much, however my knowledge of Virgin Media's infrastructure is limited to the half of it that was Telewest, and even there is not great. ADSL is indeed a different kettle of fish, often hitting a BT DSLAM (IIRC) at the exchange then performing some kind of LDAP or other directory lookup, to determine the provider, then piping the ATM level call to the providers network as the next (first outside the CP) hop in IP terms. However the big providers increasingly have their own equipment in the exchange, making this model obsolete, and most of the small provders piggy-back big ones. Anyway I am not a network engineer (although sometimes my ISP thinks I am) so that's all a bit vague and don't quote me on it etc...
Doubtless Google, if they wished, could fish a unique ID from your browser fingerprint and a postcode from your use of Google maps and pinpoint you exactly.
On 17/05/2011 10:31, River Tarnell wrote:
In article4DD16A1B.6050309@googlemail.com, Richard Farmbroughrichard@farmbrough.co.uk wrote:
Depends on the ISP, and, moreover, it depends on the granularity of information they provide. Most ADSL ISPs seem to enjoy churning IP addresses every 24 hours (possibly small hours resets of their exchange equipment). Many geo-attempts I've seem simply use ISP's registered addresses (hence eveyone lived in Woking, at one point). Smart reading of the traceroute will often give almost street level location but I doubt many people do that.
I believe "often" here means "for Virgin Media cable users".
The way VM's network is set up means it's easy to locate users to (roughly) city level, and often better if you can work out the subdivisions they use within those areas.
For ADSL users, it makes no sense to assign IP addresses or do routing based on the user's physical location, because all traffic has to go through BT (or another wholesale provider) in London anyway, and the route it takes after it gets to BT is not visible in traceroute. So, when the user connects the ISP just assigns the first available IP with no regard to location.
It's possible that there is an ADSL ISP that still assigns IPs based on physical location, but I've never seen one.
I'm fairly certain (and I've said this before) that for this reason it's impossible to do city-level geolocation of UK users in any useful way.
Even worse, MaxMind (the service Wikimedia uses) tries to "guess" the location of ADSL users, and usually fails. For example, when I used Andrews& Arnold as my ISP, MaxMind decided I lived in Arnold, Notts. There was no indication that this location was guessed or might be inaccurate, so it's not even possible to fall back to UK-wide geolocation for ADSL users.
- river.
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