Values for namespace in xmlns attribute

I see the below document using 2 namespaces

**

<root>
<h:table xmlns:h="http://ift.tt/Zgg410">
  <h:tr>
    <h:td>Apples</h:td>
    <h:td>Bananas</h:td>
  </h:tr>
</h:table>
<f:table xmlns:f="http://ift.tt/1jt69Ri">
  <f:name>African Coffee Table</f:name>
  <f:width>80</f:width>
  <f:length>120</f:length>
</f:table>
</root>

**

Question:-

Is there any naming convention for namespace or should it be only a valid http address?

For example:- In the below example I have given "test1" & "test2".Is it valid?

<root>
<h:table xmlns:h="http://test1.com">
  <h:tr>
    <h:td>Apples</h:td>
    <h:td>Bananas</h:td>
  </h:tr>
</h:table>
<f:table xmlns:f="http://test2.com">
  <f:name>African Coffee Table</f:name>
  <f:width>80</f:width>
  <f:length>120</f:length>
</f:table>
</root>