Arp tables - the ghost in the machine – Applied Motion RS-232 User Manual
Page 274
274
920-0002 Rev. I
2/2013
Host Command Reference
ARP Tables - The Ghost in the Machine
ARP, which stands for “Address Resolution Protocol”, is a low-level router function that enables traffic to be
correctly routed on the Ethernet network. It is handled automatically by the router and is normally transparent to
the user.
All network devices need to have two things: a MAC ID and an IP address.
•
The MAC ID is a unique identifier that is assigned to the chip on the network interface device. You
can think of it as a network serial number.
•
The IP address is just that - an address. Like a street address on your house. IP addresses can be
changed - MAC ID’s cannot.
The following diagram shows a basic network. Note that each device has both a MAC ID and IP address.
The router maintains an ARP table, which is really just a list that matches MAC ID’s to IP addresses. An entry is
created for every device on the network.
It should be noted that computers maintain a local ARP table as well, tracking other devices they’ve
interacted with. This is an important point because the ARP table on a PC will typically refresh more frequently
than those on a network router or switch.
So why do we care? Your application will probably require changing the IP address of a drive. The ARP
table must then be refreshed to show the same MAC ID with a different IP address. This is usually not an issue if
the drive is directly connected to the PC used to configure it, because the local ARP table will likely refresh quickly
enough to catch the new IP address and re-establish a connection.
The problem comes when the drive is connected through a router during configuration. In this scenario it is
entirely possible for IP address changes to happen more frequently than the ARP table can refresh itself. Most
routers do not allow users to refresh the ARP table directly, as this poses a significant network security risk. The
router must actually be rebooted to force a reset of the ARP table and allow a connection with the new IP address.
Obviously this is not an ideal solution.
For this reason we recommend that all configuration be performed while directly connected to a PC. Do not
use a router for drive configuration. Once an IP address is assigned the drive may be placed on the plant network
without worry.
NOTE: If you find that you are changing IP addresses often and the connection becomes unreliable, it may
be necessary to force a refresh of your PC’s local ARP table. This can be accomplished by opening a command
window and using the command
arp -d. You must have administrator privileges on your PC to do this.
MAC ID: 08:A4:C3:10:0E:00
IP: 192.168.1.100
MAC ID: A2:FB:3D:21:7A:01
IP: 192.168.1.101
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MAC ID: 03:C8:11:2B:DE:02
IP: 192.168.1.102
ARP TABLE
MAC ID: 08:A4:C3:10:0E:00 <--> IP: 192.168.1.100
MAC ID: A2:FB:3D:21:7A:01 <--> IP: 192.168.1.101
MAC ID: 03:C8:11:2B:DE:02 <--> IP: 192.168.1.102