Router knows it all?

I have broadband internet at home. The WAN port on my router has a public address, and all the hosts on my LAN have private addresses. The router performs NAT so that we can all access the internet (well over TCP and UDP anyway).

Anyway, my router has a built-in HTTP daemon which provides a web page for configuring the router. When I go into "LAN statistics", it tells me what hosts are on my LAN. It tells me their IP address, their MAC address, and their host name.

First thing I'm curious about is how the router knows what's sitting on the LAN. My first thought was that maybe it just looks at the DHCP leases it has given out, but then I tried configuring one host's IP address manually and it still came up in the host list.

Another thought I had was that maybe it just pings all the addresses in the network address range. Only problem here though is that my address range is, so it would have to ping about 16 million hosts, and I doubt that that's what it's doing.

Another thought I had was that maybe it looks at the MAC address table in its internal switch?

Does anyone know a home broadband internet router typically determines what hosts are on the LAN?

My second question pertains to how a computer knows whether there's an ethernet cable plugged into the NIC (i.e. how it knows that it's on a LAN). For instance, in Windows, I have a little icon in the bottom right corner; the icon is two little machines, and it represents my NIC network connection. When there's no ethernet cable plugged in, there's an X through the icon. When I plug a cable in (e.g. a cross-over between two laptops), then all of a sudden the X disappears and it instantly knows it's on a LAN. How is this? Also, after a second or two, it pops up a little balloon box saying whether it's 10 or 100 Mbps. Again, how does it know this?

Last thing I want to ask about is the host name. How does it get the hostname (e.g. "family-pc" or "laptop-james")? Is there some sort of layer 2 or layer 3 protocol for getting a host's name? Does anyone know a way of intercepting the hostname request in Windows?

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Tomás Ó hÉilidhe
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Each packet that goes through the Router, contains a source IP address inside it. Therefore, each time a Computer tries to send data through your Router; the Router is made aware of that Computer.

Quite possibly.

Depends on the company which made the Router. Each company might use different methods.

The NIC detects the presence of electricity flowing through it. What a miracle :-) Next, Windows notifies you (graphically) that the network card is no longer receiving electricity from a network cable.

Also, after a second or two, it pops

Auto-negotiation. It's a feature of network cards which determines the highest possible speed available, using the speed of both ends of the cable link.

Because a NetBIOS name has been set on each of these Computers.

Is there some sort of

Yes, it's called NetBIOS. It stands for Network Basic Input Output System. Microsoft created it.

Does anyone know

You can intercept the request by reading it with a type of program called a Port Sniffer.

If you wish to block these requests, then you can do so by setting up firewall rules to block NetBIOS traffic. If you use your research skills here, then you'll be able to find which TCP or UDP ports NetBIOS uses :-)

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Let's say we've got two laptops with NIC cards. If you stick a cross-over cable between the two of them, then they istantly know that they're on a LAN and the X disappears over the icon.

Obviously one way of knowing your on a LAN is just by putting a volt meter across the ethernet port and seeing if there's any traffic. But I'm talking about a situation where there was no prior communication between the two laptops, so neither of them should have been transmitting. Yet still, they know instantly that they've been connected to another machine.

I've done a port scan on my machine from within the LAN. I've got two open ports, 21 (ftp) and 3389 (Remote Desktop). Note that I don't have either of 137,138,139,445 open. So how's the router getting my computer's name. . . ?

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Tomás Ó h=C

There's a defined pattern that's sent on an idle link so that the status of the connection can be determined. This is obviously necessary for autonegotiation to work.


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