ARP requests on my net?

[Mike McCarty]

Les Mikesell wrote:
> On Tue, 2006-04-04 at 21:02, Mike McCarty wrote:
> 
>> >
>>
>>>ARP is *only* used to determine MAC addresses.  MAC addresses are *only*
>>>used within a subnet.
>>
>>Thank you. I thought I was beginning to lose my mind,
>>or had completely missed the boat on TCP/IP.
> 
> 
> TCP works over all kinds of media.  Some kinds are point-to-point

Yes, AIUI, TCP is layer 4. IP, Frame Relay, SLIP, PPP, etc.
can form the layer 3.

> like serial links where whatever you put into one end comes
> out in only one place at the other.  Ethernet is a broadcast
> media where all stations on the subnet could see what any of
> them send, but they don't want to.  The MAC address lets
> everything that doesn't want your packet ignore it efficiently,
> and to put the MAC address in the packet the sender must
> first find the one corresponding to the TCP address via ARP.

So my Linux machine is asking for router's MAC address so it
can dump packets destined for the router? That might make sense
on a 10 Base 2, yes, because everyone would see all messages
(that didn't collide, that is :-)  But the message is coming
from IP, because it knows its own IP address. Why would IP be
putting layer 2 addresses into messages?

HTTP, SMTP, FTP, etc.
TCP
IP (ICMP)
LAN/PPP/Frame Relay/ATM or etc.
physical

In this case, the LAN protocol is "ethernet", which needs to
know its own MAC, and that of its gateway. Anything not matching the
MAC should be dumped. With a semi-intelligent board the board itself
should dump packets not destined for its MAC. Is it the case that
layer 1 is asking for its gateway MAC? Somehow, this looks like
mixed layers to me. It looks like IP is asking for a MAC which
it doesn't need. Or does IP need the MAC of the destination
to instruct layer 1 where to send to the gateway? Is it presuming
that the gateway (router) may have gone down, and another device
with a different MAC may have taken over, and been assigned the
same IP via DHCP?

Mike
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