What is 127.0.0.1, and how do you peg the memorable part, 127?

127 → "tank" (t·n·k).

127.0.0.1 is localhost — the loopback address every machine uses to talk to itself; traffic to it never leaves the computer. The lead octet 1·2·7 is /t/·/n/·/k/ → tank: picture a tank parked in your own driveway, going nowhere. (127 is also the largest value of a signed byte.)