  The vulnerability of networks to targeted attacks is an issue of widespread
interest for policymakers, military strategists, network engineers and systems
biologists alike. Current approaches to circumvent targeted attacks seek to
increase the robustness of a network by changing the network structure in one
way or the other, leading to a higher size of the largest connected component
for a given fraction of nodes removed. In this work, we propose a strategy in
which there is a pre-existing, dormant spare capacity already built into the
network for an identified vulnerable node, such that the traffic of the
disrupted node can be diverted to another pre-existing node/set of nodes in the
network. Using our algorithm, the increase in robustness of canonical
scale-free networks was nearly 16-fold. We also analysed real-world networks
using our algorithm, where the mean increase in robustness was nearly
five-fold. To our knowledge, these numbers are significantly higher than those
hitherto reported in literature. The normalized cost of this spare capacity and
its effect on the operational parameters of the network have also been
discussed. Instances of spare capacity in biological networks, termed as
distributed robustness, are also presented.
