Are IP addresses unique, in other words is it possible for two
different people, from two different organization to have the same IP
address? If so, how?

Hello zole-ga,

This is sort of a tricky answer. 99.9% of the time, it can not be done
from different organizations in any sort of legal manner. However, it
can be made to look like you have the same IP as someone, although you
really do not have the same IP.

http://en.wikipedia.org/wiki/IP_address

"The actual assignment of an address is not arbitrary. An
organization, typically an Internet service provider, requests an
assignment of a netblock from a registry such as ARIN (American
Registry for Internet Numbers). The network number comprises a range
of addresses which the organization is free to allocate as they wish.
An organization that has exhausted a significant part of its allocated
address space, can request another netblock."

It may be possible to have the same IP at two different organizations,
but the address wouldn't get routed through the internet because all
the hosts in the middle "play by the rules" for assigning numbers. If
a packet from an outside host made it to a network with that thinks it
owns the address as well, it would get dropped by that organization's
router.

However, it IS possible if someone were to "spoof" the IP. Spoofing is
making your computer pretend it has a cartain IP address. So, they
really don;thave the same IP address, but for all intensive purposes,
to the average computer user, it looks that way.

If this answer requires further explanation, please request
clarification before rating it, and I'll be happy to look into this
further.
Nenna-GA
Google Answers Researcher

---

Clarification of Answer by nenna-ga on 23 Jun 2004 08:02 PDT

Hello again, I apoligize for the typos in my answer. I'm starting to
realize my keyboard (esp. my spacebar) may be biting the dust and it's
time to change it out.

Nenna-GA
GAR

For ease of understanding, there are two different types of IP
addresses. Public and Private. The Public IP addresses are those that
are unique and are typically used over the internet when your computer
exchanges data with another computer (this includes activites such as
browsing, etc.). However, since there are (actually "were" in IP
Version 4) limited number of Public range IP addresses, users inside a
network were allocated private range addresses. These private
addresses were then masked to a public range IP address so that if
these private addresses need to talk to other computers outside their
network and over the internet, they used the public IP address.
FYI, there are three Classes (not discussing the other classes as they
are out of context) of IP address ranges (depending on how big your
network is). These classes are
Class A   1.0.0.0 - 126.255.255.255
Class B   128.0.0.0 - 191.255.255.255
Class C   192.0.0.0 - 223.255.255.255
(I am not discussing "netmasking" here to keep this simple).

Each of the 3 IP network Classes have addresses reserved for private networks. 
Class A   10.0.0.0 to 10.255.255.255 
Class B   172.16.0.0 to 172.31.255.255 
Class C   192.168.0.0 to 192.168.255.255 

Thus, let say, a company A has been allocated IP address range
192.0.0.0-192.255.255.255 and there's a company B with
128.0.0.0-128.255.255.255. These were Public IP ranges. However, lets
say each of these companies have a million users each. For these
users, one approach would be to given them the private range for Class
A i.e. 10.0.0.0 to 10.255.255.255.
The machines, called gateways/routers then mask these Private IP
address to the public ones the company already has and therein stems
the use of a router and communication over the Internet.

Thank you for your rating. I'm always glad to help.

Nenna-GA

When will we start using IPv6?