How to execute webservice inside DNN site.

HttpModules are basically classes that can exist in either in Web site's source code or in a part of class library. They are integrated into the life cycle of every demand run during the ASP.NET engine. Normally they catch into events in the request life cycle and can respond to real demand or adjust the output in the Response object before it is sent to the client.

Global.asax file can also do many tasks which httpModule can do. The comparative distinction between them is that,
Standard functionality can be summarized in an httpModule and circulated among different web sites.
The benefit of the Global.asax is, easy to work with Session level events.

You will need to follow these steps to execute webservice inside DNN site,

1) Make a virtual directory in IIS 6.0 for web service folder.
2) Take a copy of the BIN files from the DotNetNuke/bin to the webservice/bin folder.

Now under webservice folder, Edit web.config file and insert the following under System.web
<httpModules>
<clear />
</httpModules>
It’ll clear out the httpModules that DNN loads, therefore let webservice to execute on it's own.