NIS has a 64K limit on the number of entries a NIS map can contain. If only service and printer contexts are created for each object (host or user), that limit will be reached when the number of users or hosts exceeds 7K. If additional contexts are created for hosts or users, as is usually the case, the upper 64K limit will be reached with far fewer hosts or users.
FNS solves this problem by automatically creating new maps after an old map has reached its maximum size. Each new map is identified by adding a numeric suffix to the map's name. For example, when a second fns_user.ctx map is created it is given the name fns_user_0.ctx. If a third map became necessary it would be given the name fns_user_1.ctx. As additional maps are created, the number is incremented each time.