User domain targeting
You can set line items to target ads only to users who visit your site from an IP address that is associated with an Internet service provider’s domain or subdomain. You can specify top-level domains, such as .edu, and .gov, and subdomains, such as stanford.edu, and usa.gov. You can also use country domains such as .uk and .jp.
Only one fragment is allowed along with the top-level domain, country domain, or top-level and country domain together. For example, you can use google.com or google.co.jp, but not www.google.com, http://www.usa.gov, or www.psychology.berkeley.edu.
Learn how to set line item targeting.