Suppose I have multiple network cards connected to my PC, how can I know which card is used for communication. For example, if I use
ping 192.168.x.xxx
I would like to know which card (and which IP address) used for communication.
The command
netstat
will tell us about the active connections. I am not interested in active connections, but I would like to know which card will be used for next connection. That is, if we use ping which card (or IP address) will be used ?
Try route print; it'll show you the OS's table of networks and which network interface it'll use for each, or where there's multiple cards on the one network the weighting it'll use in distributing the traffic. I've only got one card in this machine so this isn't very interesting:
===========================================================================
Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...00 12 34 56 78 9a ...... Intel(R) 82566DM Gigabit Network Connection
===========================================================================
===========================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 123.45.67.1 123.45.67.137 10
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
234.254.0.0 255.255.0.0 123.45.67.137 123.45.67.137 20
123.45.67.0 255.255.255.0 123.45.67.137 123.45.67.137 10
123.45.67.137 255.255.255.255 127.0.0.1 127.0.0.1 10
123.45.255.255 255.255.255.255 123.45.67.137 123.45.67.137 10
224.0.0.0 240.0.0.0 123.45.67.137 123.45.67.137 10
255.255.255.255 255.255.255.255 123.45.67.137 123.45.67.137 1
Default Gateway: 123.45.67.1
===========================================================================
Persistent Routes:
None
so e.g. from the second line traffic to 127.x.x.x will use the loopback interface but traffic to multicast and everything else will use the network card. I'm not sure how to read this programattically, though, sorry.
netstat would tell you - the local address listed in netstat would be the ip address of the network card you are using
Lets take a snippet of my own netstat output on windows - i'm connected to my lan over 192.168.1.127, and a crossover lan connection over 10.0.0.2
Proto Local Address Foreign Address State
TCP 10.0.0.2:56702 ARTEMIS:ms-wbt-server ESTABLISHED
TCP 127.0.0.1:19872 Athena:49172 ESTABLISHED
TCP 192.168.1.127:2869 192.168.1.254:49565 TIME_WAIT
TCP 192.168.1.127:56523 stackoverflow:http ESTABLISHED
10.0.0.2 is the adaptor on the local crossover, connected to ARTEMIS, which is running a RDP server, 192.168.1.127 is talking to my router, which is 192.168.1.254, and stackoverflow. Note also that local connections on 127.0.0.1 are represented.
on linux you may need to use
netstat -n
i only have one port active in my linux box, but it'll look something like - for an ip address 192.168.1.127
tcp 0 52 192.168.1.127:22 121.7.51.154:56383 ESTABLISHED
The card used is determined by the IP address configured on that card. It is worth noting that it is not generally valid to configure IP addresses on the same subnet to different physical interfaces. (Yes there are exceptions to this, but they are generally very specific, and unlikely to be useful to you at this level).
Which physical card is associated with which logical name within your operating system is a thornier issue. The easiest way to determine this is unplug all but one of the network cables and see which IP address you can still communicate on.
What operating system are you using, for certain OS types there are sometimes easier ways of determining which card has been unplugged - Windows, for example, provides visual feedback via the system tray icons.
