A country code second-level domain is a second-level domain to a country code top-level domain. Such a domain may be reserved by the domain name registry for the registration of third-level domains or assigned to a third party as a subdomain.
Many country code domain registries implement domain name classes at the second level underneath their ccTLD, such as are present in the original generic top-level domains com, net, and org, which were intended for commercial entities, network operators, and non-profit organizations, respectively.
Many countries implement additional classes. For example, the United Kingdom (uk) uses co.uk for commercial purposes and ac.uk for academic registrants.
Brazil (br) has a high number of restricted second-level domains, as of 2011 amounting to 67; they range from com.br for commercial activities, vet.br for veterinarians, to wiki.br for wikis.