Top 20 Network Engineer Interview Questions With Answers
Top 20 Network Engineer Interview Questions & Answers | Network Engineer Prep 2025
Are you preparing for a Network Engineer interview? This video/blog covers the most important 20 questions every network professional should know. From OSI & TCP/IP models, VLANs, subnetting, to routing protocols (OSPF, EIGRP, BGP), firewalls, VPNs, QoS, MPLS, and troubleshooting network latency — we’ve got you covered!
Whether you’re a beginner or experienced network engineer, this guide provides detailed, step-by-step explanations to help you ace your interview.
- What is the OSI model? Explain each layer.
The OSI (Open Systems Interconnection) model is a conceptual framework used to understand how different networking protocols interact in a network. It has 7 layers:
- Physical Layer – Deals with transmission of raw bits over a medium (cables, switches, hubs, wireless signals).
- Data Link Layer – Responsible for node-to-node data transfer and error detection (Ethernet, MAC addresses).
- Network Layer – Handles routing of packets across networks (IP addresses, routers).
- Transport Layer – Ensures end-to-end communication and data integrity (TCP, UDP).
- Session Layer – Manages sessions/connections between applications (establish, maintain, terminate).
- Presentation Layer – Translates data formats, encryption, compression (e.g., JPEG, SSL).
- Application Layer – Interfaces directly with user applications (HTTP, FTP, SMTP).
- Difference between OSI and TCP/IP models
| Feature | OSI Model | TCP/IP Model |
| Layers | 7 | 4 |
| Layers Names | Physical, Data Link, Network, Transport, Session, Presentation, Application | Network Interface, Internet, Transport, Application |
| Concept | Theoretical model | Practical model used in real networks |
| Flexibility | More flexible | Less flexible |
| Usage | Mostly for learning & design | Used in real-world networking |
- What is a VLAN and why is it used?
A VLAN (Virtual Local Area Network) is a logical segmentation of a network at the data link layer.
Uses:
- Separates broadcast domains to reduce congestion
- Enhances security by isolating groups
- Simplifies network management
- Example: Employees in Accounting VLAN can’t directly access Engineering VLAN
- Explain subnetting with an example.
Subnetting divides a large network into smaller sub-networks to improve efficiency.
Example:
- Network: 192.168.1.0/24
- Subnet mask: 255.255.255.192 (/26)
- Subnets:
- 192.168.1.0 – 192.168.1.63
- 192.168.1.64 – 192.168.1.127
- 192.168.1.128 – 192.168.1.191
- 192.168.1.192 – 192.168.1.255
Each subnet supports 62 hosts (usable addresses).
- What is DHCP and how does it work?
DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses and network settings to devices.
Workflow:
- Discover – Client broadcasts request for IP.
- Offer – DHCP server offers an IP.
- Request – Client requests the offered IP.
- Acknowledgment – Server confirms the lease.
- What is DNS? How does name resolution happen?
DNS (Domain Name System) translates human-readable domain names (e.g., google.com) into IP addresses.
Name Resolution Steps:
- User enters URL → Browser checks cache
- Query sent to Recursive Resolver
- Resolver queries Root → TLD → Authoritative Server
- IP address returned to client → Browser connects
- Explain STP (Spanning Tree Protocol).
STP prevents loops in Ethernet networks by blocking redundant paths while maintaining network redundancy.
Key Concepts:
- Root Bridge: Central reference point
- Designated Port: Forwarding port
- Blocked Port: Prevents loop
- Uses BPDU messages to elect root and block loops
- Difference between a switch, router, hub, and bridge
| Device | Layer | Function |
| Hub | Layer 1 | Broadcasts to all ports |
| Switch | Layer 2 | Forwards frames based on MAC addresses |
| Router | Layer 3 | Routes packets between networks using IP addresses |
| Bridge | Layer 2 | Connects two LAN segments, filters traffic |
- What is NAT? Types of NAT
NAT (Network Address Translation) allows private IPs to communicate over the internet using public IPs.
Types:
- Static NAT: One-to-one mapping
- Dynamic NAT: Maps private IP to a pool of public IPs
- PAT (Port Address Translation): Many-to-one mapping (uses ports)
- What is a default gateway?
A default gateway is the IP address of a router that a device uses to send traffic outside its local subnet.
- Explain routing protocols (OSPF, EIGRP, BGP)
| Protocol | Type | Key Feature |
| OSPF | Link-state | Fast convergence, uses cost metric |
| EIGRP | Hybrid | Cisco proprietary, fast, uses bandwidth & delay |
| BGP | Path-vector | Used in internet, chooses routes based on policies |
- Difference between static and dynamic routing
| Feature | Static | Dynamic |
| Configuration | Manual | Automatic |
| Flexibility | Low | High |
| CPU Usage | Low | Higher |
| Example | ip route 0.0.0.0 0.0.0.0 192.168.1.1 | OSPF, EIGRP, RIP |
- What is a firewall and how does it work?
A firewall controls network traffic based on rules.
Types:
- Packet Filtering
- Stateful Inspection
- Proxy-based
- Next-Gen (includes IDS/IPS)
Function: Filters traffic, blocks unauthorized access, logs traffic.
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Top 20 Network Engineer Interview Questions With Answers
- Explain Access Control Lists (ACLs)
ACLs are rules on routers/switches/firewalls to permit or deny traffic based on:
- Source IP
- Destination IP
- Protocol
- Port number
Example: Deny access to 192.168.1.50:
access-list 100 deny ip host 192.168.1.50 any
- What is load balancing?
Load balancing distributes network or application traffic across multiple servers to improve:
- Performance
- Availability
- Redundancy
Types:
- Hardware (F5)
- Software (NGINX, HAProxy)
- DNS-based
- What is QoS (Quality of Service)?
QoS prioritizes network traffic to ensure high-priority applications (VoIP, video) get bandwidth over lower-priority traffic.
Techniques:
- Traffic shaping
- Traffic policing
- Prioritization using DSCP or 802.1p
- Explain MPLS and its use case
MPLS (Multiprotocol Label Switching) routes packets using labels instead of IP addresses, improving speed and efficiency.
Use Cases:
- VPNs across WAN
- Traffic engineering
- Reducing latency for critical applications
- What is VPN? How does it work?
VPN (Virtual Private Network) securely connects a device to a private network over the internet.
How it works:
- Creates encrypted tunnel
- Authenticates user
- Sends data securely as if on local network
- Types: IPSec, SSL/TLS, L2TP
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- Difference between IPv4 and IPv6
| Feature | IPv4 | IPv6 |
| Address Length | 32 bits | 128 bits |
| Address Format | Dotted decimal | Hexadecimal |
| Address Space | 4.3 billion | 3.4×10³⁸ |
| NAT Needed | Often | Not required |
| Header Complexity | Simple | More complex |
- How do you troubleshoot network latency issues?
Steps to troubleshoot latency:
- Check physical connection – cables, ports, NIC
- Ping / traceroute – identify slow hops
- Check network utilization – bandwidth, congestion
- Inspect devices – switches, routers, firewall logs
- Check DNS resolution – slow DNS can cause delays
- QoS review – ensure traffic prioritization isn’t blocking critical traffic
- Test application performance – isolate network vs app issues
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Network Engineer Interview Questions, OSI Model Explained, VLAN and Subnetting, Routing Protocols OSPF EIGRP BGP, Network Troubleshooting Tips