actually, it is due to the routing tables.
every time there's a connection made, an entry is made in the routing table
Code:
C:\WINDOWS\system32>netstat -r
Route Table
===========================================================================
Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...xx yy xx zz aa bb ...... Realtek RTL8139 Family PCI Fast Ethernet NIC - Packet Scheduler Miniport
===========================================================================
===========================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 192.168.0.1 192.168.0.4 20
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
[COLOR="Blue"]192.168.0.0 [/COLOR] 255.255.255.0 192.168.0.4 192.168.0.4 20
192.168.0.4 255.255.255.255 127.0.0.1 127.0.0.1 20
192.168.0.255 255.255.255.255 192.168.0.4 192.168.0.4 20
224.0.0.0 240.0.0.0 192.168.0.4 192.168.0.4 20
255.255.255.255 255.255.255.255 192.168.0.4 192.168.0.4 1
[B]Default Gateway: 192.168.0.1[/B]
===========================================================================
Persistent Routes:
None
C:\WINDOWS\system32>
Notice the default gateway!
All new outbound connections will route thru 192.168.0.1 to the connection on the other side (the ISP) and there can only be ONE default.
All local LAN traffic flows via 192.168.0.0 to my system NIC at 192.168.0.4 to the router at 192.168.0.1
adding another NIC to the system will create another row just like
192.168.0.0 but with the IP address assigned by the DHCP on the other end (or static ip)