site will not redirect to custome page and throw 404 error in asp.net site

site will not redirect to custome page and throw 404 error in asp.net site

I don't know this question is already asked or not. But i am stuck in one
problem.
I have one CMS application in that some page are static and some are dynamic.
When i access url using http://abc.com/abc.aspx
Now in this case if abc.aspx is cms page then it will redirect me to that
page but if it is not cms page then this will redirect me to the my custom
page http://abc.com/page-not-found.aspx
Now my question is that if i write only http://abc.com/abc then it will
didn't redirect me to the http://abc.com/page-not-found.aspx but it will
throw me the error 404 page or directory not found.
Now I have to check two things
1) if any custome page is not there then it will display
http://abc.com/page-not-found.aspx
2) when http://abc.com/page then it will also redirect me to
http://abc.com/page-not-found.aspx
Please help me our from this.
Thanks in advance.
Regards AB Vyas