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I'm connected to an open network, I can see the BSSID and the SSID but I don't think DHCP is enabled because I don't get any IP. so is there a way to find out what is it's IP remotely ? and thanks

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What operation system do you use? –  Ivo Flipse Jan 23 '11 at 14:07
You are perhaps assuming that the Access Point is also the DHCP server for the network? Do you know that to be the case? –  Linker3000 Jan 23 '11 at 15:53

3 Answers 3

(Assuming Linux system) Once you have the MAC address of the AP, e.g. via iwconfig:

$ iwconfig eth1

eth1     IEEE 802.11g  ESSID:"OSU_PUB"  
         Retry limit:7   RTS thr:off   Fragment thr:off
         Power Management:off

The AP has hw addr 00:0D:9D:C6:38:2D so you can use tcpdump to sniff for traffic from that hardware address, which usually will reveal the IP address of it as the source sooner or later:

$ tcpdump -i eth1 -s 0 -v -n ether host 00:0D:9D:C6:38:2D

13:15:49.106475 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has (00:0D:9D:C6:38:2D) tell, length 28

If the AP responds to broadcast pings you could probably send a broadcast ping to its specific MAC address to elicit a reply, but there doesn't seem to be a tool capable of doing that.

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I suppose you can turn on the promiscous mode of your wireless card and start wireshark ( http://www.wireshark.org/ ) That way you will likely be able to discover the subnet of the network.

When you see a TCP packet like this...

0000 xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
0010 xx xx xx xx xx xx xx xx xx c0 a8 01 01 c0 a8 01
0020 02 xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
0030 xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx

the c0 a8 01 01 = and the c0 a8 01 02 =

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I started wireshark but where should I find the subnet, in which packet type ? –  Peter Jan 23 '11 at 12:30
are there any baseline traffic on the network? –  bubu Jan 23 '11 at 12:38
yes lots of traffic –  Peter Jan 23 '11 at 12:44
I can only see the Mac addresses and the IPs of the PPPoE connection, because this access point is for internet access using PPPoE –  Peter Jan 23 '11 at 12:51
Can you please show us a screenshot? –  bubu Jan 23 '11 at 13:00

The simplest way is usually to do netstat -rn and see what the default gateway is set to -- 99.9% of the time, that will be your access point's IP address. And it works on Linux, OS X or Windows.

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99.9% is probably reasonable for a home or small office network where the AP is often in a broadband/cable router, but much less so on a corporate network. –  Linker3000 Jan 23 '11 at 15:52

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