Structure of the Schema

Structure of the Schema

The Schema Container is located in Active Directory under the Configuration Container. For example, the distinguished name of the Schema Container in the forest would be cn=schema,cn=Configuration,dc=mycorp,dc=com. You can view the contents of the container directly by pointing an Active Directory viewer such as ADSIEdit or LDP at it. You can also use the Active Directory Schema MMC snap-in, which splits the classes and attributes in separate containers for easy viewing, even though in reality all the schema objects are stored directly in the Schema Container.

The schema itself is made up of two types of Active Directory objects: classes and attributes. In Active Directory, these are known respectively as classSchema (Class-Schema) and attributeSchema (Attribute-Schema) objects. The two distinct forms of the same names result from the fact that the cn (Common-Name) attribute of a class contains the hyphenated easy-to-read name of the class, and the lDAPDisplayName (LDAP-Display-Name) attribute of a class contains the concatenated string format that is used when querying Active Directory with LDAP or ADSI. In the schema, the lDAPDisplayName attribute of each object is normally made by capitalizing the first letter of each word of the Common-Name, and then removing the hyphens and concatenating all the words together. Finally, the first letter is made lowercase.[*] This creates simple names like user, as well as the more unusual sAMAccountName and lDAPDisplayName. We'll specify the more commonly used LDAP display name format from now on.

[*] Names defined by the X.500 standard don't tend to follow this method. For example, the Common-Name attribute has an LDAP-Display-Name of cn, and the Surname attribute has an LDAP-Display-Name of sn.

Whenever you need to create new types of objects in Active Directory, you must first create a classSchema object, defining the class of the object and the attributes it contains. Once the class is properly designed and added to the schema, you can then create objects in Active Directory that use the class. Alternatively, if you want to add a new attribute to an object, you must first create the attributeSchema object and associate the attribute with whatever classes you want to use it with.

Before we delve into what makes up an Active Directory class or attribute, we need to explain how each class that you create is unique not just within your Active Directory but also throughout the world.

X.500 and the OID Namespace

Active Directory is based on LDAP, which was originally based on the X.500 standard created by the ISO (International Organization for Standardization) and ITU (International Telecommunications Union) organizations in 1988. To properly understand how the Active Directory schema works, you really need to understand the basics of X.500; we'll run through them next.

The X.500 standard specifies that individual object classes in an organization can be uniquely defined using a special identifying process. The process has to be able to take into account the fact that classes can inherit from one another, as well as the potential need for any organization in the world to define and export a class of their own design.

To that end, the X.500 standard defined an Object Identifier (OID ) to uniquely identify every schema object. This OID is composed of two parts:

  • One to indicate the unique path to the branch holding the object in the X.500 treelike structure

  • Another to uniquely indicate the object in that branch

OID notation uses integers for each branch and object, as in the following example OID for an object:

This uniquely references object 497 in branch The branch is contained in a branch whose OID is, and so on.

Each branch within an OID number also corresponds to a name. This means that the dotted notation, for example, is equivalent to As the names are of no relevance to us with Active Directory, we don't cover them in this book.

This notation continues today and is used in the Active Directory schema. If you wish to create a schema object, you need to obtain a unique OID branch for your organization. Using this as your root, you can then create further branches and leaf nodes within the root, as your organization requires.

The Internet Assigned Numbers Authority (IANA) maintains the main set of root branches. The IANA says of itself:

The central coordinator for the assignment of unique parameter values for Internet protocols. The IANA is chartered by the Internet Society (ISOC) and the Federal Network Council (FNC) to act as the clearinghouse to assign and coordinate the use of numerous Internet protocol parameters. The Internet protocol suite, as defined by the Internet Engineering Task Force (IETF) and its steering group (the IESG), contains numerous parameters, such as Internet addresses, domain names, autonomous system numbers (used in some routing protocols), protocol numbers, port numbers, management information base object identifiers, including private enterprise numbers, and many others. The common use of the Internet protocols by the Internet community requires that the particular values used in these parameter fields be assigned uniquely. It is the task of the IANA to make those unique assignments as requested and to maintain a registry of the currently assigned values. The IANA is located at and operated by the Information Sciences Institute (ISI) of the University of Southern California (USC).

You can find the IANA web page at

You can request an OID namespacei.e., a root OID number from which you can create your own branchesdirectly from the IANA if you like. These numbers are known as Enterprise Numbers. The entire list of Enterprise Numbers assigned by the IANA can be found at This list of numbers is updated every time a new one is added. At the top of the file, you can see that the root that the IANA uses is If you look down the list, you will see that Microsoft has been allocated branch 311 of that part of the tree, so Microsoft's OID namespace is Leicester University's OID namespace is As each number also has a contact email address alongside it in the list, you can search through the file for any member of your organization that has already been allocated a number. It is likely that large organizations that already have an X.500 directory or that have developed SNMP MIBs will have obtained an OID.

In addition to Enterprise Numbers, country-specific OIDs can be purchased as well. An organization's Enterprise Number registration has no bearing on whether it has obtained a country-based OID namespace to use. If you don't see the company listed in the Enterprise Numbers list, don't be fooled; the organization could still have a number.

For example, Microsoft has been issued the Enterprise Number, yet all of its new schema classes use a U.S.-issued OID namespace of 1.2.840.113556 as their root. The 1.2.840 part is uniquely allotted to the United States. In other words, Microsoft has obtained two OID namespaces that it can use but is choosing to use only the U.S.-issued namespace.

If you want to obtain an Enterprise Number, fill in the online form at If this URL changes, you can navigate to it from the main IANA web page.

To assist customers in using properly assigned Enterprise Numbers, Microsoft has made a part of their namespace available for use. You can obtain an Enterprise Number from Microsoft's namespace by following the instructions at At present, you send an email containing your company name, contact name, email address, phone number, schema prefix, and alternate prefix (in case your initial prefix is already taken) to [email protected] If you have an Enterprise Number from another source, you should still send an email to this address and register your Enterprise Number and prefix. This can help prevent collisions on prefix names with other companies, and also allows you to register for linkIDs used in linked attributes.

Companies who do not intend to produce schema updates for use outside of their own forests may not see a benefit in registering their prefix. The benefit for them comes into play if they find out someone else has already registered that name, or if some software vendor tried to register the name and they see it is already registered. For example, say you have the company MyCorp Financial Services that extends their schema with attributes with names such as mycorpAttrib1, mycorpAttrib2, etc. Then they purchase an application from MyCorp Software Solutions, who also chose to use attribute names of mycorpAttrib1, mycorpAttrib2, etc. MyCorp Financial Services would be in a very bad position. Their only option would be changing all previous uses of the attributes so it could be used by the application; otherwise, they can't use the application that they have purchased.

Once an organization has an OID namespace, it can add unique branches and leaves in any manner desired under the root. For example, Leicester University could decide to have no branches underneath and just give any new object an incrementing integer starting from 1 underneath the root. Alternatively, they could decide to make a series of numbered branches starting from 1, each corresponding to a certain set of classes or attributes that they wish to create. Thus, the fifth object under the third branch would have an OID of 3385.3.5.

The range of values in any part of an OID namespace for the Active Directory schema goes from 1 to 268,435,455, i.e., from 20 through 228 - 1.

This limitation has caused issues with schema extensions for some companies in Australia. Australia has the OID 1.2.36, and according to the Australia Standards document MP-75, companies may use their Australian Company Number (excluding leading zeros) to formulate their OID without needing to request an OID. Unfortunately the ACN is nine digits, so it could easily exceed the limitation listed above. This has been filed as a bug and Microsoft is aware of the issue.

To reinforce this point, let's look at a couple of examples directly from the Active Directory schema. If you open the Active Directory Schema snap-in, you can look at the schema class OIDs very easily. Navigating through the classes when we open the property page for the printQueue class, we get Figure. You can see that the unique OID is 1.2.840.113556.1.5.23. This tells us that the number is a defined part of Microsoft's object class hierarchy.

printQueue Schema class properties

Figure shows the property page for the organizationalPerson class. Here, you can see that the unique OID is very different, because within the original X.500 standard, a set of original classes was defined. One of these was organizationalPerson, and this is a copy of that class. Microsoft included the entire base X.500 classes within Active Directory.

The OID numbering notation has nothing to do with inheritance. Numbering a set of objects a certain way does nothing other than create a structure for you to reference the objects. It does not indicate how objects inherit from one another.

Let's dissect an example attribute and class to see what they contain. With that information, you will be able to see what is required when you create a new schema object.

organizationalPerson Schema class properties

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