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.
