This is the type of one IP Address as reported in IIH PDUs received from the neighbor. It tags whether the corresponding IP address entry for this neighbor is IPv4 or IPv6, which matters directly on a dual-stack IS-IS deployment where an admin needs to confirm a neighbor is actually advertising both address families rather than assuming dual-stack support just because the adjacency is up. For example, an admin expecting a dual-stack neighbor to show both an ipv4 and an ipv6 entry in this table but finding only the IPv4 entry present would know that neighbor has not yet enabled or is misconfigured for IPv6 over IS-IS.