An URL must stay alive until the end of days

Simply, i expressed in title, an URL is a resource address to provide some information. Some people very hesitate about them, a site changing its urls not only lose its pagerank and search engine reputation, also loses their bookmarked / shared resources. When other peoples comes you with an old URL for some information needs, a 404 page will appear then these people dissappointed.

Solutions

1. Use a permanent redirect header (301)

A permanent redirect header will redirect a search bot to new resource url. A permanent redirect header also stay alive for bookmarked pages.

2. Use an informational custom 404 page

Some 404 pages doing a search about incoming URL and suggests correct ones. This method may be inappropriate* for some ASP and ASP.NET users, specially using IIS 6 extensionless url on shared hosting possible via a controller 404 page.

If you change this 404 page with your own exclude logic, you can use extensionless urls and url suggest features both.