The second-level domain is the readable part of the domain name that is located immediately to the left of the dot. For example, the second-level domain in coolexample.com is "coolexample." You define the second-level domain when you register most domains. However, there are some registries that restrict the second-level domain, such as .UK domains. Domains with this extension must include .CO. For example, coolexample.co.uk.
Second-level domains can be divided into further domain levels (for example, www.one.coolexample.com). These subdomains sometimes represent different computer servers within departments, but they can be created for any purpose. More than one second-level domain name can be used for the same IP address.