TTL

• Time-to-live or hop limit is a mechanism which limits the lifespan or lifetime of data in a computer or network. Once the number of hops exceeds the TTL, the packet is dropped.

Not relevant for the CCNA

Typically, TTLs begin at 255, 128, and 64. If you are reviewing a PCAP file in Wireshark, and you're seeing TCP RESET packets with a TTL of 64, it means that packet was not routed, and was most likely blocked by a Firewall.

Metadata

OSI or TCP/IP Layer

CCNA Exam Topic

Contributors

Sources

Time to live - Wikipedia
Technical Tip: Analyzing TCP RST (Reset) packets i... - Fortinet Community