What is TTL (Time To Live)?
TTL (Time To Live) refers to the amount of time or the maximum number of iterations before certain data is discarded. In the context of the internet, TTL has two main uses:
IP Packet TTL:
In IPv4, the packet header includes a TTL field. Each time an IP packet passes through a router, the TTL value is reduced by 1. When the TTL reaches zero, the packet is discarded. Essentially, the TTL of an IP packet indicates the "lifetime" of the packet, or how many routers it can traverse. This mechanism is designed to prevent packets stuck in routing loops from endlessly circulating within the network. In IPv6, a similar field exists but is called Hop Limit instead of TTL.
DNS Record TTL:
DNS (Domain Name System), which manages domain names on the Internet, uses TTL for caching purposes. DNS servers are divided into DNS Content Servers (which manage specific domains directly) and DNS Cache Servers (which query Content Servers and deliver responses to clients). The DNS Content Server assigns a TTL (measured in seconds) to the records it manages. The DNS Cache Server uses the TTL to determine how long cached records can be reused before querying the Content Server again. If the same query is received within the TTL period, the cache server provides the stored information directly without needing to contact the Content Server again.