ARP requests on my net?

Guy Fraser guy at incentre.net
Wed Apr 5 21:51:40 UTC 2006


On Tue, 2006-04-04 at 20:59 -0500, Mike McCarty wrote:
> Edward Krack wrote:
> >  Edward Krack: 
> > 
> > 
> >>It has to know where to go. Can you drive cross country
> >>without a road map?
> > 
> > 
> > Users computer types in a URL to browse the net.
> > Users computer is config to use DNS server to resolve the
> > the name in the URL to an IP address.
> > TCP/IP uses ARP.
> 
> Eh? Why does it need the MAC? And why does it need the MAC
> of my router? My router is the one which needs to know
> MACs. AFAIK, TCP/IP uses IP, not ARP. I just went and got
> my handy-dandy "Understanding TCP/IP (tm)" manual and looked
> in the index, and ARP isn't even mentioned. That's Transmission
> Control Protocol over Internet Protocol. The layers are,
> AIUI,

If you are familiar with SCSI, a MAC address is like 
a SCSI ID.

ARP is used for Ethernet {layer 1} connectivity, IP {Layer 2} 
runs on top of layer 1, and TCP {Layer 3} runs on top of 
Layer 2. TCP/IP is often misused, the biggest example is DNS 
since DNS queries are UDP not TCP, and pings are normally 
ICMP.

Ethernet devices that want to use IP need ARP in order to 
determine what MAC address is needed to send traffic to 
another device with the IP address it needs to communicate 
with, because on an "Ethernet BUS" the devices are addressed 
using a MAC address. In order for your ethernet port to talk 
to the ethernet port on your router they need to know each 
others MAC address. The devices on an ethernet segment that 
communicate with other devices using IP normally require an IP 
stack that maps an IP to a MAC address for each IP on each 
interface on each device it needs to communicate with. On some 
devices when they run out of IP stack space, they resort to 
unicast which uses a MAC address of FF:FF:FF:FF:FF:FF and causes 
all devices on the ethernet segement to listen to the request, 
this is generally unwanted and a sign that the network segment 
has too many devices or the device is not suitable for that 
large a network.

Back in the day we used to manually map IP's to MAC addresses 
using /etc/ethers. I noticed that the man page "man ethers" 
is still around, but don't know if the file will be used. If 
it is still usable it could reduce the number of ARP requests 
if you statically assign the maps in /etc/ethers.

Example:

--- /etc/ethers ---
00:11:95:0b:cc:28 172.17.205.1
--- end ---

Of course I guessed the IP of "router". ;^)

Have a good day now ya hear.






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