You work for a company that gives laptop to all employees, if you are not from the technology department, you’ll not have access to the administrator area, this normally means, among many other things, that you’ll have a fixed ip and you can’t change it.

Let’s say your fixed ip is: 192.168.5.10

Now imagine that you need to travel on business and you’ll need to connect in many networks during the travel. The airport hotspot, the client you’ll visit and the hotel.

Probably all networks you’ll visit will have a DHCP server, which would give you an available ip in the their network if your computer had DHCP set, but you have a fixed ip address and you can only use the network 192.168.5.x.

Most part of the client software that does network connection, isn’t flexible enough to switch the configuration if you are in a different network. In this case, in the hotspot, hotel or client, it would do nothing, since your gateway isn’t the same as configured in your machine, of course, unless the gateway of this new network supported ARP Spoof technique.

If those gateways had this feature, it would be totally different, the client would just join the network transparently, without having to modify a line in the laptop’s configuration.

Here is how the magic works:

It’s a man in the middle attack, the gateway will listen to all ARP Requests, whenever a client look for a different gateway, it automatically become this gateway you are looking for if possible, answer with an ARP Reply, that says I am your gateway and here is my MAC Address, from that moment on, the user is able to communicate in the network.

It’s the perfect world, you would never have to change the configuration of your computer and would join all networks without problem.

In the other hand, your work travel could be horrible, most of the networks wouldn’t be accessible and you would come back totally outside of what’s going on, not to think about the amount of unread emails.