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HowTo: Connect to an APC device with an unknown IP address

Have you ever been handed a device that has previously been configured by someone else and the IP details have disappeared?

We recently came across such a situation and thought we would share the trick we used to gain access to the device, which in turn allowed us to setup a new IP address.

This configuration was carried out on an APC AP7921, we wanted to assign IP 192.168.0.1

Firsty, find out the mac address of the unit, this should be on the back, our example 00 c0 b7 36 5c 62

Connect directly to the device using an Ethernet cable (cross over if required)

Having now found the mac address, we need to tell the arp table what the IP to MAC association should be

Windows command
arp -s 192.168.0.1 00-c0-b7-36-5c-62

Linux command
arp -s 192.168.0.1 00:c0:b7:36:5c:62

Now use ping with a packet size of 113 bytes which will assign the IP set in the ARP command

Windows command
ping 192.168.0.1 -l 113

Linux command
ping 192.168.0.1 -s 113

Now you should be able to connect using telnet or http and set the new IP address.  If you don’t know the password then you will need to use the reset button which usually requires a pin or paper click.

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