You do not strictly need another web site's permission to link your site to theirs, but it's a good idea to get permission anyway. If you do link to another Web site without permission, be sure to link to its home page. There have been some disputes about links that bypass a site's home page, because this practice may cut into the linked site's advertising revenues.
Web site publishers have also objected when other sites go beyond linking and use framed content. For example, a site may use three frames on its page, one of which displays text and images from another site. This practice confuses viewers about who owns what content, and it may violate copyright laws. In addition, you should never use a linked site's trademark on your site without permission.
Finally, you should always avoid direct linking — the process of taking another site's graphics or multimedia content by embedding its URL in your own Web page. Direct linking allows a site to use another site's copyrighted material without attribution — a clear violation of both Internet ethics and copyright law.