What is the purpose of the TTL field within an IP packet header?

clears an unreachable route from the routing table after the invalid timer expires
prevents regular update messages from inappropriately reinstating a route that may have gone bad
removes an unreachable route from the routing table after the flush timer expires
limits the period of time or number of hops a packet can traverse through the network before it should be discarded
used to mark the route as unreachable in a routing update that is sent to other routers