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... and I open a browser....

Who decides which network connection to use? Or is it intelligent or random? Is this decision made for each request, for each application session, or for each something else?

Does this mean that if I want to connect a second (or third) connection to the Internet, I should restart all my apps or I should log off and log back in?

I'm using Windows.

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what do you mean by "connections to the Internet"? Multiple ISPs? –  Keith Sep 1 '11 at 5:31
    
Yes multiple ISPs. –  William C Sep 1 '11 at 6:26
    
To one router? or two routers? What is your network topology? –  Keith Sep 1 '11 at 6:47
    
@Keith: Two network adapters, two ISPs, no routers. –  William C Sep 2 '11 at 4:01

3 Answers 3

up vote 1 down vote accepted

IP routes have "metrics", which can be used as an order value for otherwise equivalent routes. Most operating systems try to assign those intelligently – for example, if you can access the Internet over both wired Ethernet and wireless WiFi, you will have two "default" routes, but the wired one will have a lower metric and will be preferred.

$ ip -4 route
default via 192.168.1.254 dev eth0  proto static
192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.115  metric 1
192.168.1.0/24 dev wlan0  proto kernel  scope link  src 192.168.1.183  metric 2

(This is not a very good example, since both eth0 and wlan0 are connected to the same network, with the same default gateway. With multiple ISPs, you would see two default routes.)

On Linux you can list routes by ip route or route -n; on Windows – route print.

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Hi. Do you perhaps know how often these metrics are remeasured/recalculated? In Windows? –  William C Sep 2 '11 at 4:07
    
Only when the user changes them, or when the interface disappears/reappears. There is no measuring involved, just a few simple rules (which I have no idea of). –  grawity Sep 2 '11 at 5:06

When I tried to use ForceBindIP and opened Firefox without forcing it to use one of the connections, it used the last one that an app was forced to use.

I believe Windows makes the last used network connection to default for apps to use when not being forced to use a specific one, at least that's what happened to me.

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Hosts have a single default route. Generally it will use only one. A router can be configured for failover, but still any connection will use only one path.

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Yes multiple ISPs. –  William C Sep 1 '11 at 6:26

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