Airodump - find subnet?

[informatech]

ok so i'm fairly competent with airodump, aireplay, aircrack, and can crack any type of wep encryption on my DD-wrt router. now getting a key is one thing, and I know about mac filtering and such, but how do I go about figuring out the subnet settings of a given network?

I know that with a dump of encrypted packets from airodump, the mac is NOT encrypted, but how about ips? what tool under the backtrack suite would I use to figure out whether the gateway for a given AP is...

192.168.1.1
192.168.2.1
192.168.x.1
10.10.10.1
etc

To my best knowledge I would boot up windows and attempt to sift through and filter packets via wireshark, but is there a way to look at it via the backtrack suite.

also, do i have to first decrypt the packets via (i forget the tool) or can I go from there?

[imported_wyze]

Scapy / Wifizoo can do this.