DNS also has a caching mechanism set by a time-to-live
(TTL) value. The longer the TTL, the longer the domain name/IP address pair is stored
in the cache. If the TTL is 0, then the IP address is never cached.
However, phonebooks and DNS differ by the fact that a server, such as temp.evil.org,
can change its IP address at any time to any value, while Rich cannot simply tell the
phone company to change his number to any value at any time. If Rich could change his
number on the fly, he could play a prank at his high school, like this:
Rich: Hey! How??™s it going?
Worst Enemy: Why are you saying hi? You hate me, cuz I??™m dating the girl you like.
Rich: No, man. That was so yesterday. I??™m so over her. Let??™s go out tonight.
Worst Enemy: Ah. OK? What??™s your number?
Rich: Look it up in the phonebook. It??™ll be there.
At this moment, Rich would change his phone number to 911-1234. Later that night,
his ???worst enemy??? would look up his number and dial it. The phone conversation might
go like this:
911 operator: Hello, 911. What is your emergency?
Worst Enemy: Umm??¦ Ahh??¦ Is Rich there?
911 operator: No. This is 911.
???click??? (Worst Enemy hangs up)
???Ring, ring??¦???
Worst Enemy??™s Parents: Hello?
911 operator: Hello.
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