This read-only value carries additional data for a router link whose meaning also depends on the link's type field. An admin reads it together with the link type and link ID to fully interpret a given router link entry, since its role changes depending on whether the link is point-to-point, to a transit network, or a stub network. For example, for a link to a stub network this field holds that network's subnet mask rather than an address, unlike its role for other link types.