Equivalence portioning is a technique for testing 
equivalence classes rather than undertaking exhaustive 
testing for each value of the larger class.
An equivalence class is a subset of data that is 
representative of a larger class.
For example:
1.If an input condition specifies a range, then there will 
  be one valid and two invalid equivalence classes are 
  defined as follows:
  If the Name text field should be 5 to 10 alpha 
  characters. Then instead of checking 6 times Name text 
  field ,make one class with range <5-10> which will be 
  called valid class and another class will be invalid 
  class which will be less than and greater than the range.

2.If an input condition requires a specific value, then one 
  valid and two valid classes are defined.
  e.g. Input data is 6 digit number, then valid class is 
  <6 digit> and invalid classes are (<6) &( >6)

3.If an input condition is a Boolean then one valid and one 
  invalid equivalence classes are defined.
  e.g. True or False.

The aim is to minimize the number of test cases require to 
cover these input conditions.

Testing the application or field with a different set of data within a specified range.

Ex: User name should accept 2 to 6 character means we have to crate the Test case for only 2 to 6 ranges not more than that.