A field in the Internet Protocol version 4 (IPv4) header that specifies the length of time in seconds that the datagram can live in the Internet system.The maximum length of time is 255 seconds (2 8 -1, with 0 not considered, as it is the official time of death), or 4.25 minutes. From the instant the IP datagram enters the Internet, each gateway and host that acts on the datagram decrements the TTL by at least one second, although the time it has possession of the datagram generally is much less. When the TTL reaches 0, the datagram is declared dead and is discarded. The TTL mechanism prevents packets from wandering the Internet for eternity, at which point they would have no value, and would only contribute to overall network congestion. Over time, the TTL field has been redefined to indicate, as an option, the number of hops (i.e., routers) through which the packet travels. In effect, the TTL is a hop count, anyway.The default TTL is 64. See also datagram, gateway, header, host, Internet, IPv4, MPLS, and packet.
(1) (Time To Live) A parameter in a network packet that sets a time limit to its validity. In order to prevent an IP packet from propagating endlessly through the network, the value in the TTL field is reduced by each router. When TTL reaches 0, the packet is discarded. In the DNS system, which converts hostnames to IP addresses, responses use a TTL field to keep the IP address in the user's cache for a set time. If a company is preparing to change its IP addresses, it can lower the time in the TTL field before the actual change is scheduled. If the address is then cached in the user's computer, it is valid for only a short time, and the company's name server will be queried more frequently. See DNS and DNS rebinding.
(2) (Transistor-Transistor Logic) A digital circuit composed of bipolar junction transistors (BJTs). Widely used in all variety of electronic applications, especially prior to CMOS circuits becoming popular, TTL superseded the earlier RTL (resistor-transistor) and DTL (diode-transistor) logic designs, which used more power. In TTL, transistors are used to both isolate inputs and perform the logic switching. A "TTL" designation on a circuit input or output indicates a digital circuit rather than analog. See 7400 series, ECL, I2L and bipolar transistor.
(3) (Through The Lens) Refers to a single-lens reflex camera that lets the photographer view the scene through the same lens that captures the image. "TTL metering" means that the light is measured from behind the lens to determine the correct shutter and flash settings.