This IpAddress-typed field is described as the network mask for the VI-scoped static translation, again noting the mask should be zero for one-to-one mappings. An admin uses a wider mask here to translate a subnet of addresses behind a particular virtual interface rather than a single host. For example, an admin remapping an entire /29 block reachable through one VI would set this to 255.255.255.248 instead of leaving it at zero.