A hostname is the human-readable name used to find a system on a network, often a domain or subdomain such as cdn.example.com. It is the label that clients resolve through DNS into an IP address, allowing browsers, APIs, and software updates to reach the intended server.
In cyber security, hostnames matter because they sit at a trust boundary. Many applications hard-code them in code, scripts, and documentation, so a hostname can outlive the service behind it. If the name expires, is repointed, or is re-registered by another party, old clients may connect to an unexpected server. Attackers may abuse this with domain takeover or traffic interception, while defenders reduce risk by inventorying external hostnames, avoiding hard-coded dependencies, and monitoring for stale or abandoned names.