Yum looks in the files in /etc/yum.repos.d/ to determine the URLs it should use to find its repositories.
Do you have a file called /etc/yum.repos.d/core.repo? It looks like there may be a problem with the file, have a look at it and see if it includes a "baseurl=XXX" line, and see if you can navigate to the URL with a web browser.
What distro are you using?
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