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I'm not a English speaker, but I'll try my best make it clear: If your laptop connects to a unknown local network (through a wireless or wired router) and there's no DHCP service at all, and nobody would tell you the configuration of this network, how can you find out this network's IP and its subnet mask (even more the gateway's IP)?

For what I've known, most net-related software or tools work when you have configured your IP and successfully connected to it, but now I don't even know a IP to connect to it, that means most of tools won't work for you to detect anything else right?

This has confused me for a long time. I'm sure there must be a way to find out what the IP and subnet mask is. Should I set to a visit an unknown LAN? Please help!

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Putting the network interface in promiscuous mode will allow you to pick up ARP broadcasts which will tell you which IP addresses are used, but the subnet mask will still be hit-or-miss since only DHCP packets would contain information about that.

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Thanks for your quick answer,sovled the IP thing is fine,but without a smask address,I won't able to communicate with this Lan right? And there're numberless of the possible SMask address! (allthrough it wound be most likely the common one that can be guessed by the size of the network). –  JeffLyn Aug 5 '12 at 19:59
    
It could cause trouble, yes. But there's only 30 or so possible values, and only a small subset of those would make sense. The bigger issue is the default gateway, but a long enough poll time and some clever guessing can usually get that. –  Ignacio Vazquez-Abrams Aug 5 '12 at 20:01

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