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TCP/IP Network Access Layer Troubleshooting – MCQs

1.
A host cannot communicate with its local switch. Which layer is primarily responsible?
Network Access Layer
Transport Layer
Application Layer
Internet Layer
The Network Access Layer handles physical connectivity and frame delivery on LANs.
2.
An ARP request fails. What could be a probable cause?
Incorrect MAC address or VLAN mismatch
TCP checksum error
DNS misconfiguration
Port 80 blocked
ARP resolves IP to MAC addresses; incorrect MAC or VLAN can block local communication.
3.
A NIC shows no link light. What is the first step?
Check the physical cable connection
Check TCP sequence numbers
Ping the default gateway
Verify DNS settings
No link light usually indicates a physical layer issue such as a disconnected cable.
4.
Switch ports are configured in different VLANs, and hosts cannot communicate. What’s the issue?
VLAN mismatch at Layer 2
Incorrect IP subnet
Port number conflict
ARP cache overflow
Hosts in different VLANs cannot communicate unless routed; this is a Layer 2 issue.
5.
Multiple collisions are observed on a half-duplex network. How can this be resolved?
Enable full-duplex on the devices
Change TCP port numbers
Flush the DNS cache
Restart the DHCP server
Collisions occur in half-duplex Ethernet; enabling full-duplex eliminates collisions.
6.
A switch cannot forward frames to a device. What table should be checked?
MAC address table
Routing table
TCP connection table
DNS cache
Switches use MAC address tables to forward frames at Layer 2.
7.
Hosts in the same LAN cannot ping each other, but IPs are correct. Likely cause?
Firewall blocking ICMP or switch issue
TCP sequence problem
DNS failure
Incorrect HTTP port
Ping failure within LAN can be caused by Layer 2 issues or firewall filtering ICMP.
8.
A wireless client cannot associate with an AP. Which layer is involved?
Network Access Layer
Transport Layer
Application Layer
Internet Layer
Wireless association occurs at Layer 2; client must authenticate and connect to AP.
9.
Duplicate IP addresses are observed on a LAN. What tool helps identify conflicts?
ARP scan or ping sweep
Traceroute
Netstat
Telnet
ARP scans or ping sweeps help detect duplicate IPs at the Network Access and Internet Layers.
10.
A fiber link shows intermittent connectivity. What could be the cause?
Dirty or faulty fiber connectors
TCP retransmissions
Incorrect HTTP request
DNS misconfiguration
Physical layer issues like dirty fiber connectors cause intermittent connectivity.
11.
High frame loss is observed on a switch. What should be checked?
Port errors and duplex mismatch
TCP checksum
DNS server reachability
Port 80 filtering
Frame loss can occur due to physical errors or duplex mismatches on Ethernet links.
12.
What layer encapsulates data into frames?
Network Access Layer
Internet Layer
Transport Layer
Application Layer
Layer 2 (Network Access Layer) encapsulates packets into frames for transmission.
13.
A host cannot resolve MAC addresses for IPs. Which protocol is involved?
ARP
TCP
UDP
HTTP
ARP resolves IP addresses to MAC addresses for local communication.
14.
Excessive collisions are reported on a hub-based network. How to fix?
Replace hub with switch or enable full-duplex
Flush DNS cache
Restart SMTP server
Disable ICMP
Collisions are normal in hubs (half-duplex). Switching or full-duplex resolves it.
15.
Ping works, but file transfer fails. Which layer could cause this?
Network Access Layer / Transport Layer
Application Layer only
DNS only
Presentation Layer
Even if ping works, faulty frames, congestion, or blocked ports at Layer 2/4 can prevent file transfer.
1.
A user cannot access a website over HTTP. Which tool can you use to verify connectivity?
Ping and traceroute
ARP
ifconfig
nslookup is irrelevant
Ping and traceroute can verify if the host is reachable and the route is correct.
2.
Emails sent are not received. Which protocol is likely involved?
SMTP
HTTP
DNS
FTP
SMTP is responsible for sending emails; troubleshooting email delivery often involves checking SMTP.
3.
A website shows “DNS not found”. What is the first step in troubleshooting?
Check DNS server settings
Ping gateway
Check MAC address
Check TCP window size
DNS failures are usually related to DNS server settings or name resolution issues.
4.
A file cannot be uploaded using FTP. Which port should you check?
21
25
53
80
FTP control commands use port 21. Connection issues may involve firewall blocking this port.
5.
A user reports that HTTPS websites are not loading. What could be a cause?
Port 443 is blocked
Port 80 is blocked
DNS is irrelevant
ARP table corruption
HTTPS uses port 443; blocking or firewall rules can prevent access.
6.
TCP connection fails to establish. Which handshake step might have failed?
SYN/SYN-ACK/ACK
ARP resolution
DNS query
ICMP ping
TCP requires a 3-way handshake (SYN, SYN-ACK, ACK) for connection establishment.
7.
UDP packets are not received. What is a likely cause?
Firewall blocking UDP port
TCP handshake failure
DNS failure only
ARP table empty
UDP is connectionless; packet loss or firewall blocking is a common cause.
8.
Which TCP feature ensures data arrives in order?
Sequencing
Checksum
Port number
ICMP
TCP assigns sequence numbers to segments for proper ordering at the receiver.
9.
A slow data transfer over TCP can be caused by:
Congestion control
DNS resolution
ARP cache
MAC addressing
TCP congestion control can slow transmission if the network is congested.
10.
TCP port 80 is unreachable. Which application is affected?
HTTP
SMTP
DNS
DHCP
HTTP uses TCP port 80; if blocked, web traffic is disrupted.
11.
Ping fails to a remote host. Which layer is primarily involved?
Internet Layer
Transport Layer
Application Layer
Network Access Layer
Ping uses ICMP, which operates at the Internet Layer.
12.
Incorrect subnet mask can cause:
Communication failure with hosts outside the subnet
TCP checksum errors
HTTP page load issues only
MAC collisions
Subnet masks define network boundaries; wrong masks prevent proper routing.
13.
Traceroute shows timeouts at a particular router. What is likely the cause?
Firewall or ACL blocking ICMP
TCP port closure
DNS misconfiguration
Incorrect MAC
Routers may block ICMP replies for security, causing traceroute timeouts.
14.
A host cannot communicate with a default gateway. Which layer should be checked first?
Internet Layer
Application Layer
Transport Layer
Presentation Layer
IP addressing and routing are handled by the Internet Layer.
15.
Incorrect IP address assignment causes:
Network communication failure
TCP sequence error
DNS only failure
Port conflict
IP addresses must be unique; duplication or misassignment prevents communication.

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