Geralmente adicionamos rotas ao servidor que informam que o próximo salto é um determinado endereço IP. Ele usará o cartão que está na mesma sub-rede no próximo salto.
Você não disse qual SO, mas eu vejo freebsd na sua tag, então vou assumir isso.
Você tem que essencialmente fazer um roteador freebsd adicionar as rotas estáticas. Para mais detalhes do que está abaixo, visite freebsd.org usando este link . As informações a seguir são de lá.
30.2.2. Configuring a Router with Static Routes
Contributed by Al Hoang.
A FreeBSD system can be configured as the default gateway, or router, for a network if it is a dual-homed system. A dual-homed system is a host which resides on at least two different networks. Typically, each network is connected to a separate network interface, though IPaliasing can be used to bind multiple addresses, each on a different subnet, to one physical interface.
In order for the system to forward packets between interfaces, FreeBSD must be configured as a router. Internet standards and good engineering practice prevent the FreeBSD Project from enabling this feature by default, but it can be configured to start at boot by adding this line to /etc/rc.conf:
gateway_enable="YES" # Set to YES if this host will be a gateway
To enable routing now, set the sysctl(8) variablenet.inet.ip.forwarding to 1. To stop routing, reset this variable to 0.
The routing table of a router needs additional routes so it knows how to reach other networks. Routes can be either added manually using static routes or routes can be automatically learned using a routing protocol. Static routes are appropriate for small networks.
Para exemplos de configuração e mais detalhes, visite este link em freebsd.org