You would need to capture at the router, or at a hub between your computer and the router (which works as a tap). Although consumer routers typically don't capture packets, some can be flashed with OpenWRT or other firmware that can do that. For the hub approach, your best bet is an old 10Mbps hub. Modern switches (sometimes called hubs) won't work. See -http://wiki.wireshark.org/HubReference for details.