IPAddr#loopback?

# File tmp/rubies/ruby-3.4.1/lib/ipaddr.rb, line 279
def loopback?
  case @family
  when Socket::AF_INET
    @addr & 0xff000000 == 0x7f000000 # 127.0.0.1/8
  when Socket::AF_INET6
    @addr == 1 || # ::1
      (@addr & 0xffff_0000_0000 == 0xffff_0000_0000 && (
        @addr & 0xff000000 == 0x7f000000 # ::ffff:127.0.0.1/8
      ))
  else
    raise AddressFamilyError, "unsupported address family"
  end
end