Technically, TTL is theoretically measured in seconds. RFC 791 states:

Time to Live: This field indicates the maximum time the datagram is allowed to remain in the internet system. If this field contains the value zero, then the datagram must be destroyed. This field is modified in internet header processing. The time is measured in units of seconds, but since every module that processes a datagram must decrease the TTL by at least one even if it process the datagram in less than a second, the TTL must be thought of only as an upper bound on the time a datagram may exist. The intention is to cause undeliverable datagrams to be discarded, and to bound the maximum datagram lifetime.

However, as time passed and speed increased, the practical meaning for TTL has changed from seconds to number of hops the packet has encountered.

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Last Modified: November 28, 2000