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the destination represents. For IP: - Iff it is a host route, RTF_LOCAL and RTF_BROADCAST indicate local (belongs to this host) and broadcast addresses, respectively. - For all routes, RTF_MULTICAST is set if the destination is multicast. The RTF_BROADCAST flag is used by ip_output() to eliminate a call to in_broadcast() in a common case; this gives about 1% in our packet-generation experiments. All three flags might be used (although they aren't now) to determine whether a packet can be forwarded; a given host route can represent a forwardable address if: (rt->rt_flags & (RTF_HOST | RTF_LOCAL | RTF_BROADCAST | RTF_MULTICAST)) == RTF_HOST Obviously, one still has to do all the work if a host route is not present, but this code allows one to cache the results of such a lookup if rtalloc1() is called without masking RTF_PRCLONING.