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If I am plugged into a network with no DHCP and no reachable 'sample' hosts where I can peek in the configs. how do I discover/guess the network IPs range, netmask? Gateway IP address?

P.S. Don't suggest bruteforcing 192.168.*.* with various popular netmasks :))

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3 Answers

up vote 6 down vote accepted

If you were to run a packet sniffing program like wireshark or tcpdump you will likely see ARP brodcast requests that would be something like:

"Who has 192.168.1.12, tell 192.168.1.10"

From these broadcast messages you can probably infer what the network is.

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In addition to these other very good ideas, you could listen for CDP traffic. Cisco Discovery Protocol. However, there are a couple of misconceptions you've got.

  1. It's not a good practice, but a single broadcast domain can have more than one IP subnet.
  2. You don't have to have a single gateway on a network. You can have many. For a given host, only one will be the default, and that's usually what can get you out to the internet, but not always. In some cases, there's no router at all - the only way you can get out is via application-level proxies.
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You can try using SLP (or some other discovery protocol) to discover hosts, printers, etc. on the network if you have an idea of what devices may be on the network.

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SLP uses TCP and, I guess, it requires the networking to be configured in order to set a valid value to the 'source' field in the header. –  kolypto Feb 24 '11 at 21:46
    
It can use UDP for discovery, and may work the zeroconf addresses, depending on the hardware responding. –  MikeyB Feb 25 '11 at 14:38
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