When an organization moves its DNS servers to new IP addresses, the NS (Name Server) records must be updated. The NS record defines which DNS servers are authoritative for a domain. Ifthese records still point to the old IP addresses, clients will continue to query the outdated servers, leading to connectivity issues.
Breakdown of Options:
A). AAAA - This record maps a domain name to an IPv6 address. Since the issue is with DNS resolution, not IP versioning, this is incorrect.
B). CNAME - A CNAME (Canonical Name) record is used for domain aliasing, not for defining authoritative name servers.
C). MX - Mail Exchange (MX) records direct email traffic to the correct mail server, which does not impact general website accessibility.
D). NS - Correct answer. NS records must be updated to reflect the new authoritative DNS servers.
Reference:
CompTIA Network+ (N10-009) Official Study Guide - Domain 1.3: Explain the purpose and properties of DNS records.
RFC 1035: Domain Names - Implementation and Specification

When two devices on different subnets are unable to communicate, but can communicate with other devices on their own subnet, the issue is most often related to routing. Devices on different subnets require a default gateway to route traffic between networks.
If the default gateway is incorrectly configured, the device won't know how to reach other subnets.
Faulty cables (Option B) or duplex mismatches (Option C) would likely cause connectivity issues even within the local subnet, which is not the case here.
VLAN mismatches (Option D) are typically issues with switch port configurations and would likely cause total loss of connectivity, including within the same subnet.
# So, the most probable and logical cause is an incorrect default gateway.
Reference:CompTIA Network+ N10-009 Official Study Guide - Objective 2.4: "Compare and contrast routing technologies."

802.1X is a port-based network access control (PNAC) protocol that provides an authentication mechanism to devices wishing to connect to a LAN or WLAN. It is widely used for secure network access, ensuring that only authenticated devices can access the network, whether they are connecting via wired or wireless means.
802.1X works in conjunction with an authentication server, such as RADIUS, to validate the credentials of devices trying to connect.References:CompTIA Network+ study materials.

Port 22 is used for Secure Shell (SSH), which enables encrypted remote login and command execution on network devices.
Port 23 = Telnet (unencrypted)
Port 80 = HTTP
Port 123 = NTP
From Andrew Ramdayal's guide:
"SSH uses port 22 to provide secure command-line access to devices such as switches and routers. Unlike Telnet (port 23), SSH encrypts session traffic, making it the preferred method for remote administration."

The server's subnet mask is 255.255.255.224 (/27), which covers IPs from 192.168.100.224 to
192.168.100.255. However, the router only recognizes 192.168.100.0/24, indicating a mismatch between the server's subnet and the router's network.
Correct mask for the /24 network is 255.255.255.0, allowing 256 IPs from 192.168.100.0 to 192.168.100.255.
This mismatch would result in routing issues, especially with the gateway outside of the subnet range.
# Reference:
CompTIA Network+ N10-009 Official Objectives: 5.2 - Given a scenario, troubleshoot common wired connectivity issues.