An apparatus for a multi-domain computer system includes multiple virtual system domains, each having an associated, independent routing instance. A virtual switch is created to forward packets between the domains. The virtual switch is configured to operate substantially as if it were a physical layer-2 switch. The virtual switch and the virtual domains are connected together by virtual interfaces, where the virtual switch, the virtual domains, and the virtual interfaces are all created in software. Further, protocols such as ARP, IP, and/or IPv6 operate over the virtual interfaces in substantially the same way that they would over Ethernet. Also, each domain has a software driver that behaves like an Ethernet driver, and has a 48-bit Ethernet-like address that is used for the physical address.