Time-to-live (TTL) is a value in an Internet Protocol (IP) packet that tells a network router whether or not the packet has been in the network too long and should be discarded. For a number of reasons, packets may not get delivered to their destination in a reasonable length of time. For example, a combination of incorrect routing tables could cause a packet to loop endlessly. A solution is to discard the packet after a certain time and send a message to the originator, who can decide whether to resend the packet. The initial TTL value is set, usually by a system default, in an 8-binary digit field of the packet header. The original idea of TTL was that it would specify a certain time span in seconds that, when exhausted, would cause the packet to be discarded. Since each router is required to subtract at least one count from the TTL field, the count is usually used to mean the number of router hops the packet is allowed before it must be discarded. Each router that receives a packet subtracts one from the count in the TTL field. When the count reaches zero, the router detecting it discards the packet and sends an Internet Control Message Protocol (ICMP) message back to the originating host.
The default Windows 95/98 TTL value is 32 hops. Some users recommend changing this to 128 if you have difficulty reaching certain sites.
The ping and the traceroute utilities both make use of the TTL value to attempt to reach a given host computer or to trace a route to that host. Traceroute intentionally sends a packet with a low TTL value so that it will be discarded by each successive router in the destination path. The time between sending the packet and receiving back the ICMP message that it was discarded is used to calculate each successive hop travel time.
Using the multicast IP protocol, the TTL value indicates the scope or range in which a packet may be forwarded. By convention:
- 0 is restricted to the same host
- 1 is restricted to the same subnet
- 32 is restricted to the same site
- 64 is restricted to the same region
- 128 is restricted to the same continent
- 255 is unrestricted