Route types:

It is important that the set of required and optional attributes depend on the route type. The most important route type is unicast. It describes real paths to other hosts. As a rule, common routing tables contain only such routes. However, there are other types of routes with different semantics. The full list of types understood by Linux-2.2 is:

• unicast -- the route entry describes real paths to the destinations covered by the route prefix.
• unreachable -- these destinations are unreachable. Packets are discarded and the ICMP message host unreachable is generated. The local senders get an EHOSTUNREACH error.
• blackhole -- these destinations are unreachable. Packets are discarded silently. The local senders get an EINVAL error.
• prohibit -- these destinations are unreachable. Packets are discarded and the ICMP message communication administratively prohibited is generated. The local senders get an EACCES error.
• local -- the destinations are assigned to this host. The packets are looped back and delivered locally.
• broadcast -- the destinations are broadcast addresses. The packets are sent as link broadcasts.
• throw -- a special control route used together with policy rules (see sec.8, p.). If such a route is selected, lookup in this table is terminated pretending that no route was found. Without policy routing it is equivalent to the absence of the route in the routing table. The packets are dropped and the ICMP message net unreachable is generated. The local senders get an ENETUNREACH error.
• nat -- a special NAT route. Destinations covered by the prefix are considered to be dummy (or external) addresses which require translation to real (or internal) ones before forwarding. The addresses to translate to are selected with the attribute via. More about NAT is in Appendix C, p..
• anycast -- (not implemented) the destinations are anycast addresses assigned to this host. They are mainly equivalent to local with one difference: such addresses are invalid when used as the source address of any packet.
• multicast -- a special type used for multicast routing. It is not present in normal routing tables.