What is the purpose of TTL field in ip header?

Answer 1

It's a time-to-live field designating that the packet is OK to forward from one device to another for a certain amount of time. If the packet gets caught in a routing loop, it won't just go back and forth forever. If that were allowed to happen, many other packets would be doing the same thing, just being mis-routed back and forth between the confused devices, until the available bandwidth on the link was saturated.

The TTL assures that the packet will not be forwarded by the very next routing device that reads the packet's TTL field and sees that its TTL has expired. The packet would be discarded at that point.

Answer 2

The TTL field is between 0-255, and the TTL is decremented one value for each hop (each router the IP packet traverses). This means, that once an IP packet hits zero in the TTL, the router will drop the packet.

For local traffic (traffic that is never meant to leave the local subnet), a TTL of 1 is used.

If we didn't have TTL in our IP packets, they would just loop without end.

Incidentally, this is the problem with Layer 2 bridging. Layer 2 traffic has no TTL, and that is why Layer 2 network can have those pesky bridging loops.

Answer 3

The purpose is to avoid the packet from moving around the network indefinitely, in case there are routing problems - especially routing loops.The way it is dealt with is that every router decreases the TTL by one, when forwarding the packet. Once the TTL reaches zero, a router will discard the IP packet.