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Loopback address?

Posted on 2004-08-27
Medium Priority
Last Modified: 2012-08-13
Got 2 stupid questions..

Looking at my web history     within my firewall I noticed pages of  When I  went to Google it said, "Page can't be displayed."  I checked out ARIN, they came back with a loopback address. I had to look that up and it said:

 Loopback address is a special IP number ( that is designated for the software loopback interface of a machine. The loopback interface has no hardware associated with it, and it is not physically connected to a network. The loopback interface allows IT professionals to test IP software without worrying about broken or corrupted drivers or hardware.
Seaing as I am not a IT PRO..why would I have this?                                                          

Also, when I looked through the system info. within my firewall software it said, "IP address 172.xxx.x.xxx has disappeared and is no longer being protected. Hmmmm? What does the mean?

Just wondering...
Question by:chick7t7
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Assisted Solution

by:Wojciech Duda
Wojciech Duda earned 1200 total points
ID: 11911525
Each Windows machine has this so called loopback adress - it is setup automatically when you have any kind of network (LAN, Internet) set up. Just a standard procedure. This allows for example to test whether your network card responds at all to data send to it. It can also be used to block malicious sites -usually your PC checks an URL to get the adress of the server (for example www.google.com is for me). Some software (like ad-aware, spybot or your firewall) has told your PC to connect to the adress when a particular maliciuos website would be opened, therefore it does not open http:\\"malicius webiste adress"\heartbeat but This is perfectly OK. Now the other part means: Your PC had once an adress of 172.xxx.x.xxx This has changed (if you have dialup or DSL you get a new adress on each reconnect) and the old adress is no longer protected by your firewall.

Assisted Solution

boelaars earned 400 total points
ID: 11911667
With you don't test if yournetwork card responds, since it has nothing to do with your network hardware at all.

The loopback adapter is a so called virtual network card. It does not realy exist in your system, but is only installed in software. It can be used for numerous purposes. For example if you run a database server that only can accept queries through a network connection, but you don't have a network interface, or don't want it to be seen on the network at all. Then you can use the loopback adapter to connect to. You could look at it as in a way as to trick the system into believing it is networked while it's not.
It's installed on almost every system (no matter if it's unix or windows or whatever) and is best to be non-firewalled at all, since you could damage system processes by doing so.

Author Comment

ID: 11912093
mcwojtekk : You are a wealth of knowledge !

<<*This is perfectly OK. Now the other part means: Your PC had once an adress of*>>     <<*172.xxx.x.xxx This has changed (if you have dialup or DSL you get a new adress on*>>
<<*  each reconnect) and the <<old adress is no longer protected by your firewall.*>>

So that applies to LAN as well? I am assuming since my ISP is Comcast,  then I use AOL ( I like AOL features  but their search engine sucks), I must first get an IP via Comcast, then AOL? Maybe that is why the 172.xxx has "disappeared"?

 boelaars: You also, are quite the wealth of knowledge. If it wasn't for people like you, boelaars, many others from E-E and E-E, I would have given up long ago and be PC-less...

<<*It's installed on almost every system (no matter if it's unix or windows or whatever)* >>
<<*and is best to be non-firewalled at all, since you could damage system processes by* >>
<<*doing so.*>>

So this is installed on most PC's, and  it runs on its own? I have never noticed this before.  Will my firewall damage the system? Can I stop it? Should I stop it?

Thanks again!!
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Expert Comment

by:Wojciech Duda
ID: 11912247
Are you using Comcast and Aol at the same time? You mean you connect to comcast and then go to the AOL website?

The way your internet connection works, each time you reconnect you are given an andress from a range of adresses that are avaiable for the ISP. That's why the old adress has been listed as "removed", you've gotten another one as yopu reconnected. The AOL website does not give you a new adress. And yes, even in a LAN, each computer has this so called loopback device (it's a simulated device but still). It's there so each computer can check if connection to itself are possible (so each computer has the same loopback adress of pointing to itself).
LVL 16

Accepted Solution

OliWarner earned 400 total points
ID: 11912293
I'd just like to clear up a couple of things said.

"Each Windows machine has this so called loopback adress" - its not just windows - every network (TCP/IP) capable networking OS uses as the standard "this machine" IP. This means unless you do back to windows 3.1 (not 3.11) and DOS, you're never going to be able to remove this - but its not a problem! only you can access your version of at your PC... if i typed it in, i would see the equivelent on my PC...

The fact is at certain times you have a webserver running and hosting something in a /heartbeat directory.
The heartbeat page that something local on your PC is accessing is probably a way of something testing if something else it still there - that's what 'heartbeats' are generally used for.

About your other IP question, yes each ISP own a subset of IPs... eg AOL might own all of 123.456.*.*
So if you log onto AOL you might have an IP of 123.456.1.2, but the next day they may give you 123.456.5.8... depending on how their DHCP server (The machine that gives out IPs to people connecting) works.

So yes your other ISP will be handing out different A and B subnets to AOL - because they own a different one.

Expert Comment

ID: 11912306
A windows firewall will usualy leave the loopback interface alone. But if you see it blocking the interface anyway, you can safely remove the block.
It's most of the time part of the kernel or a loaded drive, so runs on its own indeed. Although I'm not perfectly familiar with windows, you should be able to stop/remove it, but if it's there I'd advice against it. It can't do any harm, while removing it could. :)

Expert Comment

ID: 11912335
loaded driveR ofcourse.
stupid typo.
LVL 16

Expert Comment

ID: 11912380
The only way to remove it would be removing the TCP/IP protocol - and if you want to carry on using your PC on the internet I dont suggest you do that...

I'd just like to reiterate that nothing can arise from leaving as it is...

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