A reservation for the server based on the MAC address means that the DHCP server will assign a specific IP address to the file server every time it requests one, based on its MAC address. This way, the file server will have a consistent IP address that users can access, without the need to manually configure it or use a separate scope. A reservation also ensures that the IP address of the file server will not be given to any other device by the DHCP server

A /23 subnet mask has 23 bits set to 1 (255.255.254.0).
The formula to calculate the number of usable host addresses in a subnet is 2^(32 - subnet bits) - 2.
For a /23 subnet:
32 - 23 = 9 bits are available for host addresses.
2^9 - 2 = 510 usable host addresses.
Reference:
Professor Messer's Network+ Study Guide
CompTIA Network+ N10-008 Study Guide

A CNAME record is a type of DNS record that maps an alias name to a canonical name, or the primary domain name. A CNAME record is used to create subdomains or alternative names for the same website, without having to specify the IP address for each alias. For example, a CNAME record can map www.example.com to example.com, or mail.example.com to example.com. Reference: CompTIA Network+ N10-008 Cert Guide, Chapter 2, Section 2.4

Firewall audit logs are records of the changes made to the firewall configuration, policies, and rules. They can help the firewall administrator to track who, when, and what changes were made to the firewall, and identify any unauthorized or erroneous modifications that could cause security issues or network outages. By consulting the firewall audit logs, the firewall administrator can investigate how the change that allowed traffic on port 80 to the web server occurred, and prevent it from happening again

The default gateway is the setting that the user most likely misconfigured on the PC that caused it to be unable to access websites by both FQDN and IP. The default gateway is a device, usually a router or a firewall, that connects a local network to other networks such as the Internet. It acts as an intermediary between devices on different networks and forwards packets based on their destination IP addresses. If the default gateway is not configured correctly on a PC, it will not be able to communicate with devices outside its local network, such as web servers or DNS servers. Reference: https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/16448-default-gateway.html