What is the rule for using 'a' or 'an' before words starting with 'n'?

What is the rule for using 'a' or 'an' before words starting with 'n'?

The rule for using 'a' or 'an' before words starting with 'n' is to use 'an' before words that start with a vowel sound, and 'a' before words that start with a consonant sound.