Why can’t the USB-Server be reached under the “old” IP address after it has been replaced?

Why can’t the USB-Server be reached under the “old” IP address after it has been replaced?

The reason frequently lies in the prompt use of an identical IP address for two devices with different MAC addresses. TCP/IP devices keep a dynamically updated ARP table which contains the associations of IP with MAC addresses. When a replacement USB-Server having the same IP address as its predecessor is placed in service, before communication is resumed the “old” ARP entry may have to be deleted in the last IP protocol processing device before the USB-Server.

ARP entries are automatically deleted after some time has passed without any data flow, so that if a longer time period passes between device replacement and resumption of communication, no manual deletion is needed. In addition, at restart the USB-Server generates special ARP packets which serve to update the ARP cache of newer TCP/IP stacks and switches.

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 In this case it is sufficient to delete the no longer valid ARP entry using the following command: arp -d [IP address]

Example 2: Communication with a USB servers takes place through one or more routers. After a unit is replaced the ARP entry for the last router before the USB-Server (gateway) must be deleted. The specific procedure can be found in the documentation from 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.

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