Which address is reserved for internal loopback functions?

Group of answer choices

$0\mathrm{.}0\mathrm{.}0\mathrm{.}0$

$121\mathrm{.}0\mathrm{.}0\mathrm{.}1$

$1\mathrm{.}0\mathrm{.}0\mathrm{.}1$

$127\mathrm{.}0\mathrm{.}0\mathrm{.}1$