IP Technical question

[Dave Howe]

Stop Common Purpose wrote:
> As I understand it, an IP address may be assigned to hundreds of
> individuals.
> 
> Is this correct?

No. only one machine on the internet can have an IP at one time - the IP
is a routing address, and is used to ensure replies to packets get where
they need to go.

Consider an IP to be like a mailing address - yes, you can have multiple
occupants sharing a single mail address, and yes, there are forwarding
services that will forward on mail sent to a specific address, but in
every case, there is only one place the mail carrier will deliver to,
and what happens to the mail after it is delivered is no longer the
carriers problem.

> If so, how is an ISP able to determine exactly the individual computer
> that accessed a particular website?

It isn't. but as a rule, IPs are assigned by ISPs on a first-come, first
served basis, and retained for as long as the customer remains connected
(once disconnected, they may be recycled rapidly, or retained for a
while longer to see if the same customer returns so that they have some
continuity of address. some (like mine) always assign the same customer
the same IP.

In any case, usually the ISP retains records of which customer account a
given IP is assigned to for any arbitrary time period; what the customer
does with it is not their concern, but they can map IP to
real-world-bill-payer.

In cases where a wifi router is in place (increasingly common) then any
machine connecting to that router will share the router's IP; the
machine will have its *own* IP (usually assigned by the router) so that
the router can tell its downstream "clients" apart. This will apply
equally to the home user's pc, his son's XBOX, the guy down the road
stealing internet access from his wifi, and the drive-by spammer who has
a wireless network finder and a laptop on the passenger seat. The IP's
traceability leads only to the bill payer, and stops there.