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Re: Ethernet does not work



> Date: Mon, 26 Nov 2001 15:19:27 -0600
> Subject: Re: [amsat-bb] Ethernet does not work
> From: Bruce Bostwick <lihan161051@earthlink.net>

Ok, this doesn't have too  much to do with satellites, but it does
seem like it should be part of everyone's basic education...

> > I want to connect my notebook B (OS is XP) with my computer A (OS is 
> > Win2k or Win NT 4 via a Ethernet LAN and cross-over cable.
> >
> > The adapters seem to work. The icons appear on the screen. The LANs are
> > activated on both. Computer A tells me, it has sent 644 packages, but
> > recieved none. B: none / none. They belong to the same workgroup:
> > "ARBEITSGRUPPE". I can ping from A to A and from B to B. But not from A 
> > to B
> > or B to A. They have different IP's 192.168.0.1 and 192.168.0.2 .

This doesn't rule out a bad cross-over cable.  I think the "link
active" (or whatever) light should be lit on both adapters.

It seems pretty easy to build a cable that doesn't work, particularly
with less-expensive crimpers.  If nothing else works, you might try
cutting the plugs off and remaking the cable (check the "link active"
lights first).

> Possibly redundant, but if you are connecting these computers via local 
> IP only and do not have a router on the LAN, you want each computer to 
> have the other's IP address as its own router address.  zum Beispiel:
> 
> Computer A:
> IP address: 192.168.0.1
> Subnet mask: 255.255.255.0
> Router address: 192.168.0.2
> 
> Computer B:
> IP address: 192.168.0.2
> Subnet mask: 255.255.255.0
> Router address: 192.168.0.1

Note that routing an IP packet is a two-step process.

First, the transmitting host determines whether the destination is
on the local network (i.e., does the destination have the same
network address as I have, or does the destination address AND the
subnet mask equal my address AND the subnet mask).  If the destination
is on the local subnet, then transmit the packet on the subnet
(e.g., on an Ethernet, ARP for the MAC address, etc.).

If the destination host is not on the local subnet, then send it
to the local router (which really ought to be on the local subnet...)

In the above configuration, Computer A and Computer B are configured
to be on the same subnet, and so they should communicate directly.
You can simply leave the router address blank (since the 
source host will transmit on the local subnet, and never need to
send the packet to the router).

> Normally, the router will have the xx.xx.xx.1 address, and all other 
> devices on the network will have addresses between xx.xx.xx.2 and 
> xx.xx.xx.254

This is solely a matter of local convention.  My convention is different
(e.g., routers have high addresses).

> (xx.xx.xx.255 is reserved for broadcast),

True.  Bad things happen if you try to assign a .255 address.

> but since there 
> is no router on this network, each computer has to know that it is 
> "routing" through the other computer directly.  This may also explain 
> why A is transmitting when you ping and B is not, since the .1 address 
> may cause A to assume it is a router for some purposes.  You might get 
> more consistent performance by assigning 192.168.0.2 to A and 
> 192.168.0.3 to B, and changing the router addresses accordingly ..

This advice is not consistent with the model I summarized above.

More free advice from,

-tjs
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