TTL

TTL, Time To Live, is a networking concept that limits data lifespan in a network. In IPv4, the TTL field carries a counter decremented by each router; if it reaches zero, the packet is discarded to prevent routing loops. The initial value is set by the sender, commonly 64 or 128. In IPv6 the equivalent is the Hop Limit, and TTL is visible in tools such as traceroute.

In DNS, TTL defines how long a resolver may cache a query result. It is a value in seconds stored with resource records (A, AAAA, etc.). A longer TTL reduces query load but risks stale data after changes. When the TTL expires, resolvers refetch the record from authoritative servers.

TTL also refers to Transistor-Transistor Logic, a class of digital integrated circuits that use bipolar transistors. Developed in the 1960s, TTL became a standard for logic families (for example, the 74xx series) and is valued for fast switching and simple interfacing. Modern designs increasingly use CMOS, but TTL remains relevant in education and certain niches.

In photography, TTL means Through The Lens metering, using a light sensor positioned behind the lens to measure exposure. In flash photography, TTL indicates automatic flash output control that uses camera–flash communication to adjust illumination. These techniques enable exposure that adapts to scene content.