Q: I can’t communicate with my router. What’s wrong?

Answer

Sometimes when configuring multiple routers in quick succession using the same computer, IP communications will fail temporarily, for all the but the first router configured. The reason for this is that the IP stack on the computer caches the MAC address of each device in an ARP table, indexed to the IP address. The first router that responds on 10.0.2.40 will have its MAC address associated with 10.0.2.40 in the ARP table. Because we ship every router with the same default address, the first time each router is accessed it is on the same IP address. The next router that uses 10.0.2.40 will have a different MAC address and when the computer tries to communicate with it, the computer may use a stale cached MAC address instead. To clear the ARP table entry for 10.0.2.40, in Windows, Linux, or OS X, enter on the command line (or DOS prompt):

arp -d 10.0.2.40

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