After Trigger and its unit test

ContactAfterTrigger.trg

trigger ContactAfterTrigger on Contact (after insert) {
    List<Contact> consToUpdate = new List<Contact>();
    
    for(Contact conInTrigger : trigger.new){
        Contact c = new Contact();//Create a new instance of an as-yet-unidentified Contact
        		c.Id = conInTrigger.Id;//Set that Contact's ID to an existing ID from the Contacts in Trigger.new 
        		c.Department = 'IT';//Change a field value on that Contact
        consToUpdate.add(c);//Add it to our List so we can update it later when all Contacts in Trigger.new have been prepared
    }
    update consToUpdate;//Perform the DML update call outside of our for-loop to avoid Governor Limits (150 DML per transaction)!
}

ContactAfterTriggerTest.cls

@isTest
public class ContactAfterTriggerTest {
    public static testMethod void testMany(){
        //Must create a test Account for the test Contacts!
        Account a = new Account(Name = 'Test Account');
        insert a;
        
        //Use a for-loop to populate a List of test Contacts
        List<Contact> testContacts = new List<Contact>();
        for(Integer i = 0; i < 2 ; i++){
            Contact c = new Contact(LastName = 'name' + i,//Use i to make each name unique
                                   AccountId = a.Id);
            testContacts.add(c);
        }
        insert testContacts;
        
        //Bulk Tests also require bulk assert tests
        for(Contact c : [Select Department from Contact]){
            System.assertEquals('IT', c.Department);
        }
    }
}