Virtual interfaces with default routes

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http://serverfault.com – My Ubuntu Server has one physical interface eth0. I want to assign 3 ip's (different subnets) to the server. I cannot use the default virtual interface creation tools like ifconfig or ip addr add (e.g. eth0:0) because for each of those 3 ip's I need to specify a default gateway. So, whether it is possible to have a bridge to which I can attach eth0 ( and multiple tap or tun interfaces with ips that I want), but each of the virtual devices can connect to subnets accessbile over eth0. (HowTos)