A host firewall is a software that controls the incoming and outgoing network traffic on a server based on predefined rules and filters. It can block or allow certain ports, protocols, or addresses that are used for communication with other servers or devices. If a server is unable to connect to another server after applying a patch, it is possible that the patch changed or added a firewall rule that prevents the connection. The administrator should check the host firewall rule and modify it if necessary to restore connectivity. Verified References: [Host firewall], [Network connection]