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 sent to it never touches a network card or leaves the computer; the operating system just hands it straight back, which makes it the go-to address for testing a server on your own machine. In fact the entire 127.0.0.0/8 block (about 16 million addresses) is reserved for loopback, though 127.0.0.1 is the one everyone uses. 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.) Peg 127 → tank.