How do I get the gateway IP address of other clients in a network?

Question

In our network, there are 2 subnets (255.255.255.0 & 255.255.252.0) and 2 IP gateways, which filters which machine will get connected to Internet thru proxy(filtered) or not. So an example of unfiltered machine, will have two IPs (bind) but using only the unfiltered gateway to the internet.

Question is, how do I obtain the gateway of these unfiltered machine (thru scanning, etc.)?

Answer 1

You have very less information for a deterministic answer.

1. Do you know the subnets with which these two network masks are to be used?
2. I assume that you do not know either of the gateway IPs -- but, only a fact that one of them is unfiltered (not proxying) to the Internet.
3. Ok, do you at least know the IP addresses (one or both) associated to the machine from which you are working?
4. If you happen to have only one IP on your machine and it is for the filtered network,
   you would at least have knowledge of the unfiltered gateway (ipconfig command on Windows, etc).
   
   • This knowledge might be useful in guessing the other gateway IP -- these IP are usually given in similar patterns
5. Further, if you know at least one machine IP (may not be yours) from the unfiltered network, it would also ease identifying the other gateway
6. Finally, what do you intend to do with the unfiltered gateway IP knowledge?
   You would still need a local IP to configure on your machine to use that gateway IP for any unfiltered access.
   
   • Now, you would need to know how IP allocation is done in your network (DHCP or static, etc).

^{Some social networking might be easier -- talk to your neighbour :-)}

Answer 2

Sounds a bit mission impossible if you don't have access to theses machines and/or the gateways/proxies.
You could try sniffing the network (with wireshark for example)... but only the hosts that are up and communicate can be seen. (You might also need to snif the 2 different networks - better ask the ITs ;-) )