creating a random number using MYSQL

This should give what you want:

FLOOR(RAND() * 401) + 100

Generically, FLOOR(RAND() * (<max> - <min> + 1)) + <min> generates a number between <min> and <max> inclusive.

Update

This full statement should work:

SELECT name, address, FLOOR(RAND() * 401) + 100 AS `random_number` 
FROM users