Confusion regarding IP address (cable vs DSL)

June 9, 2009 at 05:48:06
Specs: Microsoft Windows XP Professional, 1.699 GHz / 510 MB
I'm using cable Internet. When I type 'ipconfig' in my command prompt, I see my public IP address. Whereas my friend, who's using DSL, sees his private address when he does the same. Why?

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#1
June 9, 2009 at 05:58:22
Because, DSL is already doing NAT. They use modem/routers. If you had a router connected to your cable modem, you would see the same thing he does.

How do you know when a politician is lying? His mouth is moving.


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#2
June 9, 2009 at 06:14:37
All ISP's are doing NAT. The question relates to if the ISP is bridging their network to you or you are assigned one of the public ips in the ISPs block.

If you friend with dsl isn't behind their own router then the ISP providing DSL is bridging their network to your friend.


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#3
June 9, 2009 at 09:40:02
Thanks for the quick reply guys!
I know that all ISPs use NAT, but what I don't get is howcome I get to see my public IP address while my friend behind a DSL modem sees a private IP address?
We both are using modems; the difference is that I'm behind a cable modem and he's behind a DSL modem. The coaxial cable is hooked up to my cable modem and another cable connects the modem to the USB port of my PC. My friend has a simlar setup - the phone line is connected to the DSL modem and a twisted pair cable runs from the modem to the PC's NIC.

If you friend with dsl isn't behind their own router then the ISP providing DSL is bridging their network to your friend.

What is meant by 'bridging their network' here?


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Related Solutions

#4
June 11, 2009 at 16:32:41
As we know there are public and private ip ranges. Only public ip can be routed on the internet. Private ip can be routed on the lan.

With the shortage of ipv4 addresses some ISP's use their public ips for routing their lan traffic to the internet. They don't assign public ip to their clients. To accomplish this they give their clients private ip addresses.

This is what the case is with your friend. In your case you have a public ip given to comcast.

BTW you should be behind a router for your safety!


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#5
July 8, 2009 at 08:46:32
jojee,

Cable modems come with only public IP addresses. I guess they assume that you will only be connecting one computer in your home. Given that, there is no need for a private IP address.
Cable is shared so the more PCs you connect in your house the more bandwidth you are sharing with others in your neighborhood and everyone's speed goes down

The DSL modems actually come with NAT because you have a direct connection to the ISP. You can, with the right equiptment, can hook up as many PC in your home and you only decrease your own bandwidth.

"Bridging" is just allowing the data to pass through a device without having the device do much more than that.


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#6
July 8, 2009 at 09:34:17
Cable modems come with only public IP addresses. I

Wrong. What wanderer said above is correct:
QUOTE:
With the shortage of ipv4 addresses some ISP's use their public ips for routing their lan traffic to the internet. They don't assign public ip to their clients. To accomplish this they give their clients private ip addresses.
END QUOTE

Most, if not all ISP's use private IP addressing for their cable/xDSL clients. If you pay for a static IP (normally called a 'business class" package) you can get a public IP address. This is of course more expensive than a "consumer class" package which more often than not, gives you a private class IP address. The reason a company would want a public IP is so their IP never changes and they can run their own web server.

I guess they assume that you will only be connecting one computer in your home.

Wrong. They don't assume anything. When highspeed first became available the IPS's that offered it tried to make users pay for extra IP's for each PC/Laptop they wished to get on the internet so they could make more money. But once guys like me started showing folks how to use NAT software, they had to give up on that idea because they had no way of knowing how many PC's I was running from my NAT'd computer. Then of course hardware vendors started making SOHO routers which made the whole "multiple client PC's in your home accessing the internet through your one connection" a whole lot simpler and easier for even the least knowledgeable of users.

Cable is shared so the more PCs you connect in your house the more bandwidth you are sharing with others in your neighborhood and everyone's speed goes down

Well, you're half right in this statement. Take my house for example. Between my wife's work and home computers (She works from a home office) and my collection of computers we have over 10 PC's and 1 laptop connected to the internet. Since none of them are doing anything that eats up available bandwidth, we have no slow down issues.

However, if you, or someone in your home is up/downloading piles of large files on a regular basis (ie: software/videos/movies or music), then yes, that will slow everyone else on your LAN down because up/downloading large files really eats up bandwidth.

The DSL modems actually come with NAT because you have a direct connection to the ISP.

Wrong. A DSL modem only differs from a cable modem in regard to the type of media it connects to In the once case, a telephone line (DSL) and in the other, a coax cable (Cable). A modem is a modem is a modem. If you don't believe me, look up modem on the internet and you'll see I'm correct.

I think what you think you're talking about is a combination modem/router. A year or two back I upgraded my ADSL to a 5 Mbps package and with it came a new combo unit that is a modem/router/wireless access point. Because it has the router built in, it has NAT built in. A regular modem (ie: not a combo unit) doesn't come with, or do NAT.

"Bridging" is just allowing the data to pass through a device without having the device do much more than that.

Wrong again. Once more, I suggest you go use google and do some reading. Google "network bridge" and start by reading the wikipedia article. But don't stop there, there's a lot more info available.

I don't mean to pick on you, but misinformation is worse than NO information at all.


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#7
July 8, 2009 at 19:53:44
Kudos to Curt R for making sure no such misinformation gets out of this site!

i_Xp/VistaUser


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