response to eliminate duplicate content, and permanently storing the responses as bitstreams. Difficulties and limitations Crawlers Web archives which rely on web crawling as their primary means of collecting the Web are influenced by the difficulties of web crawling: The robots exclusion protocol may request crawlers not access portions of a website. Some web archivists may ignore the request and crawl those portions anyway. Large portions of a web site may be hidden in the Deep Web. For example, the results page behind a web form can lie in the Deep Web if crawlers cannot follow a link to the results page. Crawler traps (e.g., calendars) may cause a crawler to download an infinite number of pages, so crawlers are usually configured to limit the number of dynamic pages they crawl. Most of the archiving tools do not capture the page as it is. It is observed that ad banners and images are often missed while archiving. However, it is important to note that a native format web archive, i.e., a fully browsable web archive, with working links, media, etc., is only really possible using crawler technology. The Web is so large that crawling a significant portion of it takes a large number of technical resources. The Web is changing so fast that portions of a website may change before a crawler has even finished crawling it. General limitations Some web servers are configured to return different pages to web archiver requests than they would in response to regular browser requests. This is typically done to fool search engines into directing more user traffic to a website, and is often done to avoid accountability, or to provide enhanced content only to those browsers that can display it. Not only must web archivists deal with the technical challenges of web archiving, they must also contend with intellectual property laws. Peter Lyman states that "although the Web is popularly regarded as a public domain resource, it is copyrighted; thus, archivists have no legal right to copy the Web". However national libraries in some countries have a legal right to copy portions of the web under an extension of a legal deposit. Some private non-profit web archives that are made publicly accessible like WebCite, the Internet Archive or the Internet Memory Foundation allow content owners to hide or remove archived content that they do not want the public to have access to. Other