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I'm not in my college campus right now. In campus we had private LAN. There my computer was directly connected to a gateway. Using subnet masks I used to find out fellow nodes on my subnet. There were tens of such subnets and we used to access internet using a proxy server, it made requests on our behalf and fetched data for us. It was a 100Mbps lan so, we got speed = 12.5MBps connection speed. Things were pretty clear.

Now, I'm at home and for the first time I'm using DSL internet connection. I don't understand how this thing works? There is something called PPPoE server and my computer being called PPPoE client. If that server is acting like a proxy, how come my IP is being shown in "what is my ip?". What about my fellow nodes? Can I trace them (I mean, find their ips). Also it shows connection speed is 100Mbps. But my plan with the provider is 256Kbps. I'm not getting speed more than 356kbps but what is this 100Mbps now? When I make a request what exactly is happening ? How is this thing working and how can get a network map of this network I'm in?

EDIT:

According to the answers, the 100Mbps is the connection speed between my PC and my modem. Well, yes it is! But there are two connection icons in my system tray. And both showed 100Mbps speed. But today, something interesting happened. The other speed changed from 100Mbps to 10Mbps:

enter image description here

How did this happen? What exactly is this 10Mbps.

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How is it a security question? –  curiousguy Nov 9 '11 at 19:37
    
Strictly speaking. No its not a security question. But I thought I would get best answer here –  claws Nov 9 '11 at 19:48
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2 Answers

up vote 5 down vote accepted

[What follows is one possible organization of your ISPs internal network. From my experience (~5 years ago), it is/was relatively common at small/medium US ISPs.]

Your computer talks ethernet to your DSL modem.

The modem is connected to a DSLAM (DSL Access Multiplexer). The DSLAM forwards your traffic (possibly through one or more routers) to a BRAS (Broadband Remote Access Server) that acts as the PPPoE server.

Your DSL modem runs a PPPoE client. PPPoE is "Point to Point Protocol over Ethernet". So your modem is talking PPPoE on the network side (as opposed to ATM, etc). The PPPoE is carried to the BRAS, where it is stripped of PPP and the IP is handed off to another router heading toward the Internet.

Here's a quick sketch:

DSL Network Sketch

The PPPoE server assigns an IP address to your modem. It is not a "proxy" in the sense that it is a web proxy. All it is really doing is wrapping the IP packets from the internet and sending PPPoE to your modem. (And the reverse for outbound traffic.)

With PPPoE, all traffic is forced out to the BRAS -- you don't have any "fellow nodes", there is no "LAN", you're basically on your own subnet. Sniffing will not do you any good -- there's no traffic for other nodes coming into your modem from the BRAS.

You can't get a network map because your traffic is encapsulated (as PPPoE) as it travels over the various internal nodes of the ISPs network. You might be able to enumerate other nodes that share your IP address block, but finding a "nearby" address doesn't mean that user is close geographically or network-wise. And, since your traffic will be going through the BRAS and possibly the router, your scan could land in the logs and prompt some uncomfortable questions from a network admin.

(Even if your ISP wasn't using PPPoE, it is likely that all of the traffic from your modem would be forced up to a router ("MAC Forced Forwarding"). This makes it easier for the ISP to enforce policies, including CALEA compliance and separation of subscribers from one another.)

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wait! did you sketch that for this answer? –  claws Nov 9 '11 at 20:21
    
@claws: Yes, just a quick sketch -- that's why it's a little ugly... –  bstpierre Nov 9 '11 at 21:53
    
I've edited and added another question. Can you have a look at it? –  claws Nov 10 '11 at 9:22
    
@claws: That part of the question seems far out of scope for this site, and not a question I know how to answer. –  bstpierre Nov 10 '11 at 14:30
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PPPoE is an encapsulation protocol which is derived from PPP. PPP was designed for providing IP traffic over a serial link: it consists in messages which either transport data (i.e. IP packets) or do "administrative" things such as authenticating a user or allocating an IP address. PPPoE is a lightweight way of transporting PPP packets over Ethernet frames.

With PPP, you are effectively on a kind of subnet of your own, containing a single IP address (yours). When the PPP link has been established, the administrative packets informed your operating system of the IP address which was allocated to you, and the IP address of the ISP machine which is at the other end of the link, which acts as a router (not a proxy: that router does not "hide" your IP from the rest of the world; it just sits on the only link between you and the outside, so all IP packets must go through it). There are no "fellow nodes" on the same subnet than you.

The "100 Mbps" you see is the speed of the Ethernet link between your machine and your DSL modem. It means that whatever bandwidth you may get will be capped at 100 Mbits/s because that's all that can go over the cable which sprouts out of your computer. However, nobody says that between the DSL modem and the ISP it will still be 100 Mbits/s (and, indeed, it will not be).

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my plan with the provider is 256Kbps. That definitely is not the cable speed of any cable b/w me and server (router). Because I've an option of upgrading it up to 2Mbps. So, how are they capping my speed to 256kbps? –  claws Nov 9 '11 at 20:17
    
Since all your external traffic goes through a machine hosted by the ISP (the one at the other end of the PPP), it is trivial for that machine to enforce arbitrary delays before forwarding an IP packet in either direction. –  Tom Leek Nov 9 '11 at 20:57
    
I've edited and added another question. Can you have a look at it? –  claws Nov 10 '11 at 9:23
    
@claws: In the case of DSL, there are physical-layer methods of controlling the bit rate. It is likely that these are in use instead of throttling at the router or BRAS. –  bstpierre Nov 10 '11 at 22:19
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