Can I determine whether two IP addresses come from the same source?

In this excerpt from
Answercast #57, I examine the way that IP addresses relate and the few things
that can be referred from the numbers.

Similar IP addresses

No. So, when you take a look at an IP addresses, the number of digits isn’t
as important as what we consider to be the ‘four groups’ of digits. Each of
those can be anywhere from one to three digits long. It’s typically referred to as a
“dotted quad notation” because you basically have four numbers separated by
dots.

The problem is that in some cases you can make some inferences from
the IP addresses as to whether or not one IP address is related to another.

For example, I at home have five IP addresses.

The way they’re laid out implies that it’s not just the first two numbers that have to be correct. In reality, it has to be the first three of the four numbers that have to be identical. AND the number of numbers available in that last number (which can be anything from 0 to 255) can only be one of five specific values.

Then yes, if you know that, if you can figure that out, then you realize
that those five IP addresses all come back to me.

IP address numbers vary

Now, the problem of course is that it varies dramatically. It really does.
In some cases, it’s enough to know just the first number. There are some
allocations of IP addresses where all of the IP addresses that begin with X dot
(where X is some number, I’m not sure what the number is)… they were all
allocated to the same company. So if an IP address starts with that number,
then you know that IP address came from, or is somehow related to,
that company.

In reality, it’s much more complex than that because it definitely goes down
all the way to the scenario that I just described where it’s not until you get
all three of the first three numbers identical PLUS a limited set of
the fourth number that you can actually define that an IP address came from
the same place.

Whois lookup

There’s really, really no way for the average person to make that kind of
determination. The best you can do (and the suggestion that I will make) is
that you take each of the two IP addresses that you are concerned about and run
them through what’s called a “Whois” service.

If you go to “whois.domaintools.com/” followed by the IP address, that will
tell you what ISP owns that IP address. So you can at least see if the two IP
addresses are owned by the same ISP:

• If they’re not… well, you’ve got part of the answer to your question.

• If they are, you still don’t know.

The ISP may have assigned them to the same person; they may have assigned
them to two completely different people on two completely different locations
on the planet. There’s no way for you to know.

In fact, there’s no way that the ISP is going to tell you without a court
order. So ultimately, we can’t positively answer the question that you’re
asking.

• Can you determine that they are next to each other? No.

• Can you sometimes determine that they aren’t next to each
  other? Sometimes, yeah. Just because they’ll be owned by different
  ISPs.