Basic ASCII: Alt 33 to 126
Every printable character on a US keyboard, the letters, digits and punctuation from ! through ~, has an Alt code between 33 and 126. These are the plainest codes on the site: no leading zero, straight ASCII, unchanged since the original IBM PC character set.
Nobody memorises these to type an ordinary letter A. They matter for two other reasons: automation scripts that send keystrokes by character code, and troubleshooting when a keyboard driver or input method is misbehaving and a character will not appear the normal way.
Control characters below 32, such as Tab and Escape, are left out, since they have no visible glyph to look up.
Questions
Why would I need the Alt code for a letter I can already type?
Mostly for scripting or keyboard-driver testing, where a character needs to be sent by its numeric code rather than a keypress.
What is the Alt code for a plain space?
Alt+32, though pressing the space bar does the same thing with less effort.
Do these codes need the numeric keypad?
Yes, all Alt codes do. The number row along the top of the keyboard will not work.
What is the highest Alt code in this range?
126, the tilde. Code 127 has no visible glyph, so it is left off this page.
Does this range cover both letters and punctuation?
Yes. Alt+33 through Alt+126 covers every printable character on a US keyboard: digits, punctuation and both cases of the alphabet, all without a leading zero.