XML NAMESPACES
Using XML to define your organization's business data and process 
information can be very difficult when you have overlapping departments 
and redundant data definitions. Having two XML elements called 
CustomerInformation can be detrimental to your XML implementation if 
the two elements are not identical. XML namespaces can help resolve 
conflicts between element names.

OVERVIEW
The basic problem that namespaces solve is collision between 
identically named elements. A namespace is a set of names that exist 
within a particular domain, the namespace. A good analogy is shoe 
sizes. A size nine is not always a size nine. Not because different 
makers have different tolerances, but because a women's size nine is 
not the same as a men's size nine. These sizes exist in two different 
domains: women's and men's. These domains are analogous to namespaces.

The reason that XML namespaces exist is to help identify and resolve 
conflicts between elements that have the same name but mean different 
things. This situation can occur when elements from different DTDs are 
included in the same XML document. In this scenario, each DTD would 
maintain it's own unique XML namespace, and the resulting XML document 
would refer to its elements in the context of which namespace they 
belong.

HOW THEY WORK
Namespaces are identified within the XML document where they are used. 
An XML document may use multiple namespaces simultaneously. Each 
namespace is identified using a Unified Resource Identifier (URI). The 
URI points to an external file that defines the particular namespace 
being used. In addition to identifying the namespace resource, the XML 
document may assign a variable name to the particular namespace that 
will be used in element names to identify to which namespace they 
belong. The following example illustrates how multiple namespaces might 
be defined in an XML document:

<Customers 
  xmlns:cust="http://www.myserver.com/xmlns/customers" 
  xmlns:addr="http://www.myserver.com/xmlns/address"
> 
<cust:CustomerInformation> 
  <cust:CustomerNumber>990023</cust:CustomerNumber> 
  <addr:Address> 
    <addr:Street>9902 Broadway</addr:Street> 
    <addr:City>Chicago</addr:City> 
    <addr:State>IL</addr:State> 
    <addr:ZipCode>60612</addr:ZipCode> 
  </addr:Address> 
</cust:CustomerInformation>

In this example, two namespaces have been declared. The first one 
refers to the customers namespace and indicates that elements and 
attributes in the customers namespace will be prefixed by the tag cust. 
The second one refers to the address namespace that is identified by 
the addr prefix. 

DEFAULT NAMESPACE
In addition to declaring namespaces identified with a prefix, you have 
the option of declaring a default namespace. The default namespace 
handles the names of elements and attributes that do not contain a 
namespace prefix. The following example illustrates the use of both a 
default namespace and a named namespace:

<OrderInformation  
    xmlns:cust="http://www.myserver.com/xmlns/customers" 
    xmlns="http://www.myserver.com/xmlns/orders" 
> 
  <OrderData> 
    <cust:CustomerData> 
      <cust:Name>Joe's Corp.</cust:Name> 
      <cust:Number>99123</cust:Number> 
    </cust:CustomerData> 
    <LineItems> 
      <Item> 
        <Name>Widget Bracket</Name> 
        <Number>344-34-599834</Number> 
        <Quantity>12</Quantity> 
        <PricePer>233.50</PricePer> 
      </Item> 
    </LineItems> 
  </OrderData> 
</OrderInformation>

In this example, you can see that both the <Name> and <Number> elements 
have been used more than once. Because the default namespace has been 
declared as the orders namespace, the orders elements do not require a 
prefix. Thus the elements <Name> and <Number> in the <Item> element 
refer to the orders definition of an item name and item number. The 
<cust:Name> and <cust:Number> elements above refer to the customers 
namespace definition of a customer name and customer number.

SUMMARY
Creating XML documents and specifications that don't use overlapping or 
duplicate names can be a difficult task. This undertaking can be 
complicated even more when documents from multiple organizations are 
brought together. Using XML namespaces can help alleviate issues where 
XML elements and attributes use identical names.