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Let's assume you must work in a network but they forgot to tell you the ip address range.
# tcpdump -nnei eth1 13:46:05.577596 00:1a:73:3f:7a:9d > 00:03:6f:e1:5b:21, ethertype IPv4 (0x0800), length 74: 192.168.0.194.33387 > 184.108.40.206.53: 5597+ A? www.google.com. (32) 13:46:05.676650 00:03:6f:e1:5b:21 > 00:1a:73:3f:7a:9d, ethertype IPv4 (0x0800), length 142: 220.127.116.11.53 > 192.168.0.1 94.33387: 5597 4/0/0 ...
In this example,
192.168.0.194 is a host in the network, and
00:03:6f:e1:5b:21 is the mac address of the gateway. We don't know the ip address of the gateway yet, that would probably require waiting for an arp packet, or try guess it, or active scan.