Brian Cryer's Web Resources


1. Time To Live.

On an IP based network (such as TCP/IP), TTL is the number of hops an IP packet may make across the network. A Time To Live value that is part of each IP packet's header. Each time an IP packet passes across a router its TTL value is decremented by one, when it reaches 0 it is no longer forwarded - because it has exceeded its TTL value.

As part of DNS, TTL indicates how long a DNS record is valid for - and therefore when the address needs to be rechecked (in case it has moved).

Caching algorithms may also incorporate a TTL value, in which case it is normally the length of time something is to be cached before being discarded (or refreshed).

For more information see:

2. Transistor-Transistor Logic. Refers to digital integrated circuits that operate at discrete voltage levels, with a voltage representing logical 0 or 1. Details vary from system to system, but an example would be 0 to 0.8 volts representing logical 0 and 2.3 to 5 volts representing logical 1.