This is the type of the IP next hop address. It tags whether the associated IP Reachable Address entry's next hop is IPv4 or IPv6, a small but necessary companion field since the actual next-hop address object is stored as a generic variable-length InetAddress that needs this type field to be parsed correctly. For example, an entry with this set to ipv6 tells the router and any management tool reading the table to interpret the accompanying next-hop value as a 16-byte IPv6 address rather than a 4-byte IPv4 one.