Why after replacing a Web-IO can you no longer talk to the “new” one under the “old” IP address?

The reason for this often has to do with the close (time-wise) use of an identical IP address for 2 devices having different MAC addresses. TCP/IP stations tend to use a dynamically administered ARP table which contains the associations of IP with MAC addresses. If a replacement Web-IO having the same IP address as its predecessor is installed, before resuming communication you may have to delete the “old” ARP entry in the last device to process the IP protocol from the Web-IO.

After a certain time, ARP entries are deleted automatically with no data flow, so that, if a longer interval elapses between device replacement and the reestablishment of communications, it is possible to do without the manual deletion. Above and beyond that, when restarting, the Web-IO generates special ARP packages which have the function of updating the ARP cache of newer TCP/IP stacks and switches.

Example 1: A Windows machine communicates without use of a router with a Web-IO in the same IP subnet. In this case it is sufficient to use the following command to delete the no longer valid ARP entry: arp -d [IP address]

Example 2: Communication with a Com-Server is through one or more routers. After an exchange, the ARP entry of the last router (gateway) in front of the Web-IO must be deleted. The specific procedure can be found in the documentation supplied by the respective manufacturer.

Note: Since switches also process and save the MAC addresses of the connected devices, these must also be taken into account when replacing a device.

Additional information:

• Product Info for Web-IO
• Manual for Web-IO

[Start of page]