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Computer Networks Interview Questions & Answers
40 questions with detailed answers — for freshers and experienced candidates.
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Q1. What is a computer network?
A computer network is a group of interconnected computers and other hardware devices that are able to share resources and data. These connections can be established through cables (like Ethernet) or wirelessly (like Wi-Fi). The primary goal is to enable communication and resource sharing among devices, such as sharing files, printers, or internet access. Networks range from small local area networks (LANs) in homes or offices to large global networks like the internet, facilitating various applications from email to web browsing.
Q2. Explain the difference between a Hub, Switch, and Router.
A **Hub** is a basic networking device that connects multiple Ethernet devices, broadcasting all incoming data to all connected devices, making it inefficient. A **Switch** is smarter; it learns the MAC addresses of devices and forwards data only to the intended recipient, improving efficiency and reducing collisions. A **Router** operates at Layer 3 (Network layer) and connects different networks. It uses IP addresses to forward packets between networks, making decisions about the best path for data to travel.
Q3. What is an IP address?
An IP (Internet Protocol) address is a unique numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication. Its primary functions are host or network interface identification and location addressing. IP addresses enable devices to locate and communicate with each other over a network. There are two main versions: IPv4 (e.g., 192.168.1.1) and IPv6 (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334), with IPv6 offering a much larger address space.
Q4. What is the OSI model and why is it important?
The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven distinct layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer performs specific functions and communicates with the layers directly above and below it. It's important because it helps understand how data travels across a network, facilitates troubleshooting by isolating issues to specific layers, and promotes interoperability among different vendors' products by providing a common reference.
Q5. Name the 7 layers of the OSI model.
The seven layers of the OSI model, from bottom to top, are:
1. **Physical Layer:** Deals with physical transmission, electrical/optical signals.
2. **Data Link Layer:** Provides reliable data transfer across a physical link, handles MAC addressing.
3. **Network Layer:** Handles logical addressing (IP addresses) and routing of packets.
4. **Transport Layer:** Provides end-to-end communication, segmentation, and reassembly (TCP, UDP).
5. **Session Layer:** Manages communication sessions, establishes, manages, and terminates connections.
6. **Presentation Layer:** Translates data, handles encryption/decryption, compression.
7. **Application Layer:** Provides network services to end-user applications (HTTP, FTP, SMTP).
Q6. What is TCP/IP model? How is it different from OSI?
The TCP/IP model is a four-layer conceptual model used in the Internet Protocol Suite, defining how devices connect and communicate over a network. Its layers are: Network Access (or Link), Internet, Transport, and Application. It is different from the OSI model primarily in its number of layers and its practical implementation. The TCP/IP model is a more practical, real-world model that the internet is built upon, whereas the OSI model is a more theoretical reference model. The Network Access layer combines OSI's Physical and Data Link layers, and the Application layer combines OSI's Session, Presentation, and Application layers.
Q7. What is a MAC address?
A MAC (Media Access Control) address is a unique identifier assigned to network interfaces for communications within a network segment. It is a 48-bit (6-byte) hexadecimal number, typically displayed as six pairs of hexadecimal digits separated by colons or hyphens (e.g., 00:0A:95:9D:68:16). MAC addresses are "burned in" to the network interface card (NIC) by the manufacturer and are used at the Data Link Layer (Layer 2) of the OSI model for local network communication. Unlike IP addresses, MAC addresses are hardware-specific and generally don't change.
Q8. What is the purpose of DHCP?
DHCP (Dynamic Host Configuration Protocol) is a network management protocol used on IP networks for dynamically assigning IP addresses and other communication parameters to devices connected to the network. Without DHCP, network administrators would have to manually assign IP addresses to every device, which is tedious and error-prone, especially in large networks. DHCP automates this process, ensuring that each device gets a unique IP address within a specified range, along with subnet mask, default gateway, and DNS server information, making network management much simpler.
Q9. What is DNS and how does it work?
DNS (Domain Name System) is a hierarchical and decentralized naming system for computers, services, or any resource connected to the Internet or a private network. Its primary function is to translate human-readable domain names (like "google.com") into machine-readable IP addresses (like "172.217.160.142"). When you type a domain name into your browser, your computer queries a DNS server, which resolves the name to an IP address, allowing your browser to connect to the correct server. This makes accessing websites much easier than remembering numerical IP addresses.
Q10. What is the difference between client-server and peer-to-peer networks?
In a **client-server network**, dedicated servers provide resources and services (like file storage, printing, or web hosting) to client devices. Clients request services, and servers respond. The server is typically more powerful and manages network resources. Examples include web servers and email servers. In a **peer-to-peer (P2P) network**, all connected devices (peers) have equal capabilities and responsibilities. Each peer can act as both a client and a server, sharing resources directly with other peers without a central server. File sharing applications like BitTorrent are common examples of P2P networks.
Intermediate Level
Q1. Explain the three-way handshake in TCP.
The TCP three-way handshake is a process used by TCP to establish a connection between a client and a server before data transmission begins. It ensures that both sides are ready to send and receive data.
1. **SYN (Synchronize):** The client sends a SYN packet to the server, indicating its desire to establish a connection.
2. **SYN-ACK (Synchronize-Acknowledge):** The server receives the SYN, acknowledges it by sending a SYN-ACK packet back to the client.
3. **ACK (Acknowledge):** The client receives the SYN-ACK, acknowledges the server's SYN by sending an ACK packet. Upon receiving this, the server considers the connection established. Both sides are now ready for data exchange.
Client Server
SYN_SENT ----SYN----> SYN_RECV
ESTABLISHED <---SYN-ACK--- SYN_RECV
ESTABLISHED ----ACK----> ESTABLISHED
Q2. What is the difference between TCP and UDP? When would you use each?
TCP (Transmission Control Protocol) is a connection-oriented, reliable protocol that guarantees delivery, orders packets, and performs error checking. It uses a three-way handshake and flow control, making it suitable for applications where data integrity is critical, such as web browsing (HTTP), email (SMTP), and file transfer (FTP).
UDP (User Datagram Protocol) is a connectionless, unreliable protocol. It does not guarantee delivery, order, or error checking. It's faster and has lower overhead than TCP. UDP is used for applications where speed is more important than perfect reliability, such as streaming video/audio, online gaming, and DNS queries, where occasional packet loss is acceptable or can be handled by the application layer.
Q3. What is subnetting? Why is it used?
Subnetting is the process of dividing a single large IP network into smaller, more manageable subnetworks. This is achieved by borrowing bits from the host portion of an IP address and using them for the network portion, creating a "subnet mask" that delineates the network and host parts.
Its primary purposes are:
1. **Efficiency:** Reduces network traffic by confining broadcasts to smaller segments.
2. **Security:** Allows network administrators to isolate parts of a network for security reasons.
3. **Organization:** Makes network management easier by creating logical divisions.
4. **IP Address Conservation:** In IPv4, it helps conserve public IP addresses by allowing a single public IP to be used for multiple private subnets (with NAT).
Q4. Explain the concept of NAT (Network Address Translation).
NAT (Network Address Translation) is a method of remapping one IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device. Its main purpose is to allow multiple devices on a private network to share a single public IP address when accessing the internet. This conserves public IPv4 addresses. When a device from the private network sends a request to the internet, the router's NAT converts the private source IP to its public IP. When a response comes back, NAT translates the public destination IP back to the private IP of the original requester.
Q5. What are routing protocols? Give examples.
Routing protocols are specialized network protocols that determine the best path for data packets to travel from a source to a destination across a network. They enable routers to exchange information about network topology, available routes, and the "cost" (e.g., hop count, bandwidth, delay) associated with each route.
Examples include:
* **Interior Gateway Protocols (IGPs):** Used within an autonomous system (AS).
* **RIP (Routing Information Protocol):** Distance-vector protocol.
* **OSPF (Open Shortest Path First):** Link-state protocol.
* **Exterior Gateway Protocols (EGPs):** Used between different autonomous systems.
* **BGP (Border Gateway Protocol):** The routing protocol of the internet.
Q6. What is ARP and how does it work?
ARP (Address Resolution Protocol) is a protocol used by the Data Link layer to resolve an IP address to a physical MAC address. When a device wants to communicate with another device on the same local network, it knows the target's IP address but needs its MAC address to construct the Ethernet frame.
1. The source device broadcasts an ARP request packet containing the target's IP address.
2. The device with the matching IP address responds with an ARP reply packet, providing its MAC address.
3. The source device receives the MAC address and stores it in its ARP cache for future use, then proceeds to send the data.
Device A (192.168.1.10) wants MAC of Device B (192.168.1.20)
1. Device A broadcasts: "Who has 192.168.1.20? Tell 192.168.1.10"
2. Device B responds: "192.168.1.20 is at 00:11:22:33:44:55"
Q7. Describe the function of a Firewall in a network.
A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It acts as a barrier between a trusted internal network and untrusted external networks (like the internet). Firewalls can be hardware-based, software-based, or a combination. They protect against unauthorized access, malicious attacks, and data breaches by filtering traffic based on IP addresses, port numbers, protocols, and even application-layer content. They are crucial for maintaining network security and enforcing network policies.
Q8. What is the difference between IPv4 and IPv6?
IPv4 (Internet Protocol version 4) uses 32-bit addresses, allowing for approximately 4.3 billion unique addresses, typically represented in dotted-decimal format (e.g., 192.168.1.1). Due to address exhaustion, IPv6 (Internet Protocol version 6) was developed. IPv6 uses 128-bit addresses, providing a vastly larger address space (approximately 3.4 x 10^38 unique addresses). It is represented in hexadecimal format (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334). IPv6 also offers improved routing efficiency, better security features (IPsec built-in), and auto-configuration capabilities compared to IPv4.
Q9. Explain the concept of port numbers.
Port numbers are 16-bit integers (0-65535) used by the Transport Layer (TCP/UDP) to identify specific applications or services running on a host. When a data packet arrives at a device, the IP address directs it to the correct device, and the port number directs it to the correct application or process within that device. Well-known ports (0-1023) are assigned to common services (e.g., HTTP uses port 80, HTTPS uses 443, SSH uses 22). Registered ports (1024-49151) are assigned to specific applications, and Dynamic/Private ports (49152-65535) are used by client applications.
Q10. What is HTTP and HTTPS? How do they differ?
HTTP (Hypertext Transfer Protocol) is an application-layer protocol for transmitting hypermedia documents, such as HTML, over the internet. It is stateless and operates on port 80. HTTPS (Hypertext Transfer Protocol Secure) is the secure version of HTTP. It uses SSL/TLS (Secure Sockets Layer/Transport Layer Security) to encrypt the communication between a client (web browser) and a server. This encryption protects the data from eavesdropping and tampering, ensuring confidentiality and integrity. HTTPS operates on port 443 and is crucial for secure transactions, login pages, and protecting sensitive information on the web. The "S" signifies security.
Q11. How does a VPN work?
A VPN (Virtual Private Network) extends a private network across a public network, like the internet, enabling users to send and receive data as if their computing devices were directly connected to the private network. It works by creating an encrypted "tunnel" between the user's device and a VPN server. All data passing through this tunnel is encrypted, making it unreadable to anyone intercepting it. This provides security and anonymity, masking the user's IP address and location. VPNs are used for secure remote access to corporate networks, bypassing geo-restrictions, and enhancing online privacy.
Q12. What is the difference between Unicast, Broadcast, and Multicast?
* **Unicast:** A one-to-one communication method where a single sender sends data to a single receiver. This is the most common form of data transfer on the internet (e.g., browsing a website).
* **Broadcast:** A one-to-all communication method where a single sender sends data to all devices on a specific network segment or subnet. Routers typically do not forward broadcast traffic, limiting its scope to the local network (e.g., ARP requests, DHCP discovery).
* **Multicast:** A one-to-many communication method where a single sender sends data to a specific group of receivers who have expressed interest in receiving that data. It's more efficient than multiple unicasts for group communication (e.g., live streaming, online gaming).
Q13. Explain the concept of Latency, Bandwidth, and Throughput.
* **Latency:** The delay before a transfer of data begins following an instruction for its transfer. It's often measured as the round-trip time (RTT) for a packet. Lower latency means faster response times.
* **Bandwidth:** The maximum rate of data transfer across a given path. It's typically measured in bits per second (bps). Higher bandwidth means more data can be transferred per unit of time.
* **Throughput:** The actual rate at which data is successfully transferred over a communication channel. It's often lower than the theoretical bandwidth due to factors like network congestion, packet loss, and processing delays. While bandwidth is the capacity, throughput is the actual performance.
Q14. What is a Proxy Server?
A proxy server acts as an intermediary for requests from clients seeking resources from other servers. Instead of connecting directly to the destination server, a client sends its request to the proxy server, which then forwards the request to the destination server. The proxy server receives the response and forwards it back to the client.
Proxies are used for several purposes:
1. **Security:** Masking client IP addresses, filtering malicious content.
2. **Performance:** Caching frequently accessed content.
3. **Access Control:** Restricting access to certain websites.
4. **Monitoring:** Logging internet usage.
5. **Anonymity:** Hiding the client's identity.
Q15. What is the purpose of ICMP?
ICMP (Internet Control Message Protocol) is a network layer protocol used by network devices, like routers, to send error messages and operational information indicating, for example, that a requested service is not available or that a host or router could not be reached. It is primarily used for diagnostic purposes and to report errors. The `ping` command, which tests connectivity to a host, uses ICMP echo request and echo reply messages. `traceroute` (or `tracert`) also relies on ICMP to map the path taken by packets across a network. ICMP does not carry user data directly but is crucial for network troubleshooting and management.
Advanced Level
Q1. Describe the difference between link-state and distance-vector routing protocols.
**Distance-Vector Protocols (e.g., RIP):** Routers share their *entire routing table* with directly connected neighbors. They only know the "distance" (metric) and "vector" (next hop) to destinations. They rely on neighbors for accurate information, leading to slower convergence and potential routing loops. Updates are periodic.
**Link-State Protocols (e.g., OSPF, IS-IS):** Routers share information about their *directly connected links* (link-state advertisements) with *all* other routers in the autonomous system. Each router builds a complete topology map. They use algorithms like Dijkstra's to calculate the shortest path. Offer faster convergence, less prone to routing loops, and better scalability. Updates are event-driven.
Q2. How does TCP handle flow control and congestion control?
**Flow Control:** Prevents a fast sender from overwhelming a slow receiver. TCP uses a "sliding window" mechanism. The receiver advertises its available buffer space (window size) to the sender. The sender can only transmit data up to this window size before waiting for an acknowledgment and an updated window advertisement. If the receiver's buffer fills up, it advertises a zero window, pausing the sender until buffer space becomes available.
**Congestion Control:** Prevents the network itself from becoming overwhelmed. TCP uses several algorithms: Slow Start (exponentially increases congestion window), Congestion Avoidance (linearly increases window), and Fast Retransmit/Fast Recovery (retransmits lost packets quickly and adjusts window).
Q3. Explain the process of establishing a secure connection using SSL/TLS.
The SSL/TLS handshake establishes a secure connection:
1. **Client Hello:** Client sends supported TLS versions, cipher suites, and a random number.
2. **Server Hello:** Server responds with chosen TLS version, cipher suite, its own random number, and its digital certificate (containing its public key).
3. **Authentication:** Client verifies the server's certificate using trusted CAs.
4. **Key Exchange:** Client generates a pre-master secret, encrypts it with the server's public key, and sends it. Server decrypts with its private key. Both parties independently generate symmetric session keys from the pre-master secret and random numbers.
5. **Change Cipher Spec & Finished:** Both parties signal readiness for secure communication, sending encrypted messages with the new session key. Subsequent data is encrypted with these session keys.
Q4. How does BGP (Border Gateway Protocol) work? What are its key attributes?
BGP is a path-vector routing protocol that routes traffic between autonomous systems (ASes) on the internet. It exchanges reachability information, including a list of ASes (AS_PATH) that a route has traversed. It doesn't primarily use metrics like hop count, but rather policy-based decisions.
**Key Attributes:**
* **Path Vector:** Includes AS_PATH to prevent loops and enable policy routing.
* **Policy-Based Routing:** Allows administrators to prefer paths based on business agreements, cost, etc.
* **Incremental Updates:** Only changes are exchanged.
* **TCP Port 179:** Uses TCP for reliable communication.
* **Attributes for Path Selection:** Uses attributes like LOCAL_PREF, AS_PATH length, MED, Origin to determine best path.
Q5. Explain the concepts of connection pooling and persistent connections in web applications.
**Connection Pooling:** A technique to reuse existing network connections (especially for databases) instead of opening a new one for every request. An application requests a connection from the pool; if available, it's assigned; otherwise, a new one is created. After use, it's returned to the pool, reducing the overhead of connection establishment/teardown and improving performance and scalability.
**Persistent Connections (HTTP Keep-Alive):** Allows a client and server to keep a TCP connection open after an HTTP request/response. This enables multiple HTTP requests and responses to be sent over the same connection, avoiding the overhead of establishing a new TCP connection for every resource (HTML, CSS, images) on a webpage, thereby reducing latency and network congestion. HTTP/1.1 uses persistent connections by default.
Q6. What is a CDN (Content Delivery Network) and how does it improve network performance?
A CDN (Content Delivery Network) is a geographically distributed network of proxy servers and their data centers. Its goal is to provide high availability and performance by distributing content spatially relative to end-users. When a user requests content, the CDN directs the request to the nearest server (edge server) that can deliver it.
**Improvements:**
* **Reduced Latency:** Content served from a closer server minimizes data travel time.
* **Faster Loading Times:** Less latency leads to quicker page loads.
* **Reduced Bandwidth Costs:** Offloads traffic from origin servers.
* **Improved Availability & Redundancy:** Requests are rerouted if an edge server fails.
* **DDoS Protection:** Can absorb and mitigate DDoS attacks by distributing traffic.
Q7. Describe the role of a Load Balancer in a network architecture.
A load balancer is a device or software that distributes network traffic efficiently across multiple servers. Its primary goal is to maximize throughput, minimize response time, and avoid overloading any single server.
**Key Functions:**
1. **Traffic Distribution:** Uses algorithms (e.g., round-robin, least connections) to direct requests to the most appropriate server.
2. **Health Checks:** Monitors backend servers, removing unhealthy ones from rotation.
3. **Session Persistence/Affinity:** Ensures subsequent client requests go to the same server for stateful applications.
4. **SSL Termination:** Can handle SSL/TLS encryption/decryption, offloading this task from backend servers.
5. **High Availability:** Provides redundancy by distributing traffic, preventing a single point of failure.
Q8. What are the different types of network attacks and how can they be mitigated?
1. **DDoS (Distributed Denial of Service):** Overwhelms a system with traffic. *Mitigation:* CDNs, anti-DDoS services, rate limiting.
2. **Man-in-the-Middle (MitM)::** Attacker intercepts communication. *Mitigation:* HTTPS/SSL/TLS, strong encryption, VPNs.
3. **Phishing:** Tricking users for sensitive info. *Mitigation:* User education, email filters, MFA.
4. **SQL Injection:** Injecting malicious SQL code. *Mitigation:* Parameterized queries, input validation, WAFs.
5. **Malware (Viruses, Ransomware):** Malicious software for damage or unauthorized access. *Mitigation:* Antivirus, firewalls, IDS/IPS, regular patching, user training. Effective mitigation involves a layered security approach and continuous vigilance.
Q9. Explain the concept of Software-Defined Networking (SDN).
SDN (Software-Defined Networking) is an architectural approach that separates the network's control plane (which makes decisions about where traffic is sent) from the data plane (which forwards the traffic). In traditional networks, these two planes are tightly coupled within individual devices. Key principles include **centralized control** by a software controller, **programmability** through APIs for dynamic configuration, and **abstraction** of infrastructure. This allows for greater flexibility, agility, and automation in network management, making it easier to deploy new services, optimize traffic, and respond to changing network conditions. OpenFlow is a common protocol used in SDN.
Q10. How does QoS (Quality of Service) work in networks?
QoS (Quality of Service) refers to technologies that manage network traffic to reduce packet loss, latency, and jitter. It prioritizes certain types of traffic (e.g., VoIP, video conferencing) to ensure they receive necessary bandwidth and performance, especially during congestion.
**Key Mechanisms:**
1. **Classification:** Identifying and categorizing different traffic types.
2. **Marking:** Tagging classified traffic with a QoS label (e.g., DSCP) to indicate priority.
3. **Queuing:** Placing packets into different queues based on priority.
4. **Scheduling:** Determining the order in which packets are transmitted from queues.
5. **Policing/Shaping:** Limiting bandwidth for certain traffic or dropping packets that exceed a defined rate to prevent network overload.
Q11. What is an Autonomous System (AS) in networking?
An Autonomous System (AS) is a collection of connected IP routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet. Essentially, it's a large network or a group of networks managed by a single entity (e.g., an ISP, large corporation). Each AS is assigned a unique 16-bit or 32-bit AS Number (ASN). Routing within an AS uses Interior Gateway Protocols (IGPs) like OSPF, while routing between different ASes uses an Exterior Gateway Protocol (EGP), primarily BGP (Border Gateway Protocol), to exchange routing information and define paths across the internet.
Q12. Explain the concept of network segmentation and its benefits.
Network segmentation is the practice of dividing a computer network into multiple smaller segments or subnets, either physically (separate switches/routers) or logically (VLANs, firewall rules). This creates isolated security zones.
**Benefits:**
1. **Improved Security:** Prevents lateral movement of threats; a breach in one segment is contained.
2. **Reduced Congestion:** Limits broadcast domains, minimizing unnecessary traffic and improving performance.
3. **Enhanced Performance:** Localizes traffic, reducing overall network load.
4. **Easier Management:** Simplifies troubleshooting and policy enforcement by creating logical groups.
5. **Compliance:** Helps meet regulatory requirements by isolating sensitive data. Common methods include VLANs, firewalls, and microsegmentation for finer control.
Q13. Discuss the security implications of using public Wi-Fi networks and how to mitigate them.
Public Wi-Fi networks are often unencrypted and uncontrolled, posing significant risks.
**Implications:**
* **Eavesdropping/Sniffing:** Attackers can intercept unencrypted data.
* **Man-in-the-Middle (MitM) Attacks:** Attackers can set up fake hotspots to intercept or redirect traffic.
* **Malware Injection:** Malware can be injected into unencrypted traffic.
* **Session Hijacking:** Stealing session cookies to gain account access.
**Mitigation:**
* **Use a VPN:** Encrypts all traffic.
* **HTTPS Everywhere:** Ensure websites use HTTPS.
* **Avoid Sensitive Transactions:** Refrain from online banking/shopping.
* **Disable File Sharing:** Turn off network discovery.
* **Use a Firewall:** Keep device firewall active.
* **Keep Software Updated:** Patch vulnerabilities.
* **Be Skeptical:** Verify Wi-Fi network names.
Q14. How do container networking and Kubernetes networking differ from traditional virtual machine networking?
**Traditional VM Networking:** VMs typically get dedicated virtual NICs, connected to a virtual switch, mapping to a physical NIC. Each VM acts like a separate machine with its own IP, managed at the hypervisor level.
**Container Networking (e.g., Docker):** Containers share the host OS kernel and often share the host's network interface (bridge, host mode). Networking is lighter-weight, focusing on isolating processes. Each container can have its own IP or share the host's.
**Kubernetes Networking:** Designed for orchestration across hosts.
* **Flat Network:** All pods communicate without NAT.
* **Unique IP per Pod:** Each pod gets its own IP.
* **Service Abstraction:** Kubernetes Services provide stable IPs/DNS names for groups of pods, handling load balancing.
* **Network Policies:** Fine-grained control over pod communication. Kubernetes uses CNI plugins for implementation.
Q15. Explain the concept of a Virtual Local Area Network (VLAN).
A VLAN (Virtual Local Area Network) is a logical grouping of network devices into a single broadcast domain, regardless of their physical location or the physical port they are connected to on a switch. Devices in a VLAN communicate as if they are on the same physical segment, even if physically separated.
**Key Benefits:**
1. **Security:** Isolates traffic, preventing unauthorized access and containing breaches.
2. **Broadcast Control:** Reduces broadcast domains, minimizing unnecessary traffic and improving performance.
3. **Flexibility:** Allows easier network reorganization without physical recabling.
4. **Management:** Simplifies administration through logical grouping and policy application. VLANs are implemented by tagging Ethernet frames with a VLAN ID (IEEE 802.1Q).
Prepared by iCampusLink. 40 Computer Networks interview questions.