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IP addresses can seem confusing; email headers have them, routers have them, web pages have them... how do you sort it all out?
I've had the same IP address assigned so long by my cable company that it is virtually static. As I understand it, for the Internet to work, no other computer can have the same IP address. You have explained (I think) that one's external IP address is actually associated with one's router, not their modem. A friend assured me if I bypass my router and plug directly into my modem, my external address won't change. I tried that and could no longer access the Internet because of "DNS lookup failure." I'm guessing I could have restarted the modem or something to get it to work, but I didn't want to tinker further so switched the cables back. If I were to take my router out of the picture, how would I keep the same external IP address? Or would I?
Also, can I conclude that the originating IP address on an email is REALLY the originating IP address (as in it can't be fudged by someone else)?
Last, probably an apples to oranges question... if IP addresses are unique, how is it that thousands of different web pages share the same one?
In this excerpt from Answercast #24, I sort out all of the various IP addresses that can be assigned on a home network system, in emails, and on webpages.
Actually several good questions here.
The one assumption that your friend made, "If you plug directly into your modem your external IP address won't change." I would not make that assumption.
I think that there is no reliable way of changing your configuration that would guarantee that your IP address won't change. I don't believe that it's necessarily an IP address assigned to your modem. It may very well be assigned to the router and the act of unplugging the router could break that assignment. In general, it's really difficult to say for sure because different ISPs will do things in slightly different ways.
So, I would say that changing any of the connectivity to the internet (the router or the modem), runs the risk of having the IP address change out from underneath you. Honestly, if a static IP address is that important to you, you're just gonna have to actually pay for one from your ISP.
Now, the DNS lookup failure is an easier one to resolve.
If you were to reboot your PC, a couple of things would happen.
When you connect from your local area network (when you connect through your router), very often the DNS is routed through the router. In other words, the IP address that was assigned for your DNS server was actually the local IP address for your router, which is probably 192.168.1.1. By just moving the cable from your router to your modem, that didn't change.
So now all of a sudden, your computer is connecting and asking 192.168.1.1 to do a DNS lookup; but that IP address no longer connects to anything because you have removed the router from the picture.
So that's why I say rebooting probably would have resolved that issue.
If you were to take your router out of the picture, I don't know of a way to keep the same external IP address.
Anything that then asks for a new IP (for example, reconnecting a computer directly and rebooting it... or as simple as just rebooting the router) could potentially cause your ISP to assign you a new IP address.
So, the next part of that question: "Can I conclude that the originating IP address on an email is really the originating IP address? That it can't be fudged by someone else?"
In general, no, you cannot. It depends a lot on the email systems that are involved; but there are definitely ways to obfuscate it. Everything from:
Munching the headers;
To using botnets to send emails (so while the IP address is technically correct, it's actually the IP address of an infected machine and has nothing to do with the actual sender of the email);
To systems like Gmail or Hotmail (which may or may not actually include an IP address of the person that sent the email... the IP address that you find would be the IP addresses of the Gmail or Hotmail servers).
Sometimes, there'll be an originating IP address. Sometimes, headers can be fudged and sometimes, they can't.
And the last question, "How is that thousands of different web pages share the same one?"
IP addresses are not assigned to web pages. IP addresses are assigned to servers.
In other words, an IP address is assigned to a device:
It can be a router;
it can be a server;
it can be a PC.
On that server, you can then host one, two... thousands, of different
websites; which themselves can then house thousands of different web pages. The
IP address only gets you as far as server, the box that these sites are
actually hosted on. It's the server that takes the request and makes sure
that the right piece of content is served up; based on the actual name of the
site you're requesting, and the page that you're requesting.
Next from Answercast 24 – How do I diagnose a machine that won't boot reliably?
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