Connect to ActiveDirectory using DirectoryEntry

Question

Hi,

I want to connect with an AD server;

This is how I try:

// ldap=OU=Employees,DC=xxxxxx,DC=yyyyyyy,DC=kkk  
// port = 389;
string ldapPath = "LDAP://" + serverName + ":" + port + "/" + ldap;    
using (DirectoryEntry rootEntry = new DirectoryEntry(ldapPath))
{
                //username does not contain the prefix: CN=xxxxxx\
                rootEntry.Username = username;
                rootEntry.Password = password;

                object k = rootEntry.NativeObject;
}

Can you tell me what am I doing wrong?
I have the ADExplorer tool, and successfully connected there, without stating the server name at all ! (only domain, user and password)