When clustering machines, you make it possible for a failover node to be accessible on the same address as the primary node by having a virtual network interface. In Debian, you can add one like this: 

iface eth0:0 inet static
    address 192.168.1.75
    network 192.168.1.0
    broadcast 192.168.1.255
    netmask 255.255.255.0
    gateway 192.168.1.251

By leaving out the auto eth0:0, I prevent the interface from becoming active automatically. I can do “ifup eth0:0” to enable it. This can also be left to clustering software, but clustering software can also get in the way, so often you just want to do it by hand.