Interview questions
Networking Interview Questions
20 computer networking interview questions with model answers: OSI and TCP/IP, subnetting, DNS, DHCP, VLANs, routing and troubleshooting. For both sides.
What's here
20 Networking interview questions, grouped into 4 areas: models and fundamentals, addressing and core services, switching, routing and name resolution and troubleshooting and security. Each one comes with why it is worth asking and what a strong answer contains, so the same question can be scored the same way by different interviewers.
- Include at least one subnetting question worked out on paper; it is quick to check and hard to bluff.
- Use a troubleshooting scenario and let the candidate ask questions; the questions they ask show their method.
- Ask for commands they would run (ping, traceroute, nslookup, ip route, show commands on a switch) and what each result would tell them.
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Preparing for this interview yourself? Practise it free →These networking interview questions cover the computer networks concepts asked in interviews for network engineers, IT support, system administrators, cloud and security roles: the OSI and TCP/IP models, TCP and UDP, subnetting, DNS, DHCP, switching, routing and structured troubleshooting. Candidates can use the 'what to look for' notes as model answers; interviewers can use them to score answers and choose follow-ups.
Models and Fundamentals
The shared vocabulary of networking. Weak answers here usually mean weak troubleshooting later.
- 1
Walk me through the OSI model and name a protocol or device at each layer.
Why ask it
The OSI model is the common language for describing where a problem sits.What to look for
Physical (cables, signals), Data Link (Ethernet, MAC addresses, switches), Network (IP, routers), Transport (TCP, UDP), Session, Presentation (encoding, encryption) and Application (HTTP, DNS). Strong candidates map it to the four-layer TCP/IP model and use it to describe a real fault. - 2
What is the difference between TCP and UDP? Give examples of when each is used.
Why ask it
Basic, but it shows whether the candidate understands the trade-offs.What to look for
TCP is connection-oriented, with ordered, reliable delivery, retransmission and flow control. UDP is connectionless with no delivery guarantee and lower overhead. TCP suits web, email and file transfer; UDP suits DNS queries, voice, video and gaming, where speed matters more than retransmitting lost packets. - 3
Describe the TCP three-way handshake.
Why ask it
Underpins troubleshooting of connection failures and firewall issues.What to look for
The client sends a SYN with its initial sequence number; the server replies with its own SYN that also acknowledges the client's (often called SYN-ACK); the client sends an ACK. Data can then flow. Good answers mention the connection close sequence and what a missing SYN-ACK suggests (filtered port, no listener). - 4
What happens, at the network level, when you type a URL into a browser?
Why ask it
An open question that shows how well the candidate connects the pieces.What to look for
DNS resolution (cache, resolver, recursive lookup), a TCP connection (or QUIC over UDP for HTTP/3), a TLS handshake for HTTPS, the HTTP request and response, and routing through the default gateway. Strong candidates mention ARP for the gateway's MAC address and NAT on the way out. - 5
What is the difference between a hub, a switch and a router?
Why ask it
Checks basic device knowledge for support and junior roles.What to look for
A hub repeats every signal to every port (Layer 1). A switch forwards frames by MAC address using a MAC table (Layer 2). A router forwards packets between networks by IP address using a routing table (Layer 3). Layer 3 switches combine switching and routing.
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Addressing and Core Services
IP addressing, subnetting and the services that make networks usable.
- 6
Split 192.168.10.0/24 into four equal subnets. What are the ranges and how many usable hosts does each have?
Why ask it
Subnetting is a must-have skill and easy to verify.What to look for
Borrow two bits to get /26 subnets of 64 addresses each: .0 to .63, .64 to .127, .128 to .191 and .192 to .255. Each has 62 usable host addresses, because the first is the network address and the last is the broadcast address. Mask 255.255.255.192. - 7
What are private IP address ranges, and how does NAT let them reach the internet?
Why ask it
Almost every network uses private addressing.What to look for
RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 for private use; they are not routed on the public internet. NAT, usually port address translation, rewrites private source addresses to a public address and tracks connections using port numbers. - 8
What does ARP do, and what is an ARP-related attack?
Why ask it
Connects Layer 2 and Layer 3 and has a security angle.What to look for
ARP maps an IPv4 address to a MAC address on the local network: a broadcast request, a unicast reply and a cached entry. ARP spoofing sends forged replies to redirect traffic through an attacker. Mitigations include dynamic ARP inspection on managed switches. - 9
Explain how DHCP gives a device an IP address.
Why ask it
DHCP problems are a common support ticket.What to look for
Four messages: DHCPDISCOVER (client broadcast), DHCPOFFER (server proposes an address and options), DHCPREQUEST (client accepts one offer) and DHCPACK (server confirms). It runs over UDP, with servers on port 67 and clients on port 68. Good answers mention leases, relay agents for other subnets, and what a self-assigned address suggests. - 10
What are the key differences between IPv4 and IPv6?
Why ask it
IPv6 is increasingly common, especially in cloud and mobile networks.What to look for
IPv6 addresses are 128 bits instead of 32, written in hexadecimal, which removes the address shortage that drove NAT. The header is simplified, and routers do not fragment IPv6 packets: only the source node does. Candidates should be able to read an abbreviated IPv6 address.
Switching, Routing and Name Resolution
How traffic is segmented and moved, and how names become addresses.
- 11
What is a VLAN, and what is a trunk port?
Why ask it
VLANs are standard in any managed network.What to look for
A VLAN splits one physical switch network into separate broadcast domains for security and traffic control. Access ports carry one VLAN; trunk ports carry several, tagging frames with 802.1Q tags. Traffic between VLANs needs a router or Layer 3 switch. - 12
What is the difference between static and dynamic routing? Compare OSPF and BGP.
Why ask it
Routing knowledge separates network engineers from generalists.What to look for
Static routes are configured by hand and suit small or stub networks; dynamic protocols learn and adapt to changes. OSPF is a link-state protocol used inside an organisation; BGP is a path-vector protocol used between organisations and on the internet, with policy-based path selection. - 13
How does DNS resolution work, and what are the common record types?
Why ask it
'It's always DNS' is a running joke because DNS problems are so common.What to look for
The client asks a resolver, which checks its cache and otherwise queries root, top-level domain and authoritative servers in turn, caching results for the TTL. Records: A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail), TXT (verification and email security), NS (name servers). - 14
What is a default gateway, and what happens if it is set wrongly?
Why ask it
A simple question that maps directly to everyday faults.What to look for
The router a host sends traffic to for any destination outside its own subnet. If it is wrong, the host can reach local devices but nothing beyond the subnet. Good answers describe checking it with ipconfig or ip route. - 15
What problem does Spanning Tree Protocol solve?
Why ask it
Switching loops can take down a whole network.What to look for
Redundant links between switches create loops, which cause broadcast storms and unstable MAC tables. STP elects a root bridge and blocks redundant paths, unblocking them if an active link fails. Candidates may mention Rapid STP and features such as BPDU guard.
Troubleshooting and Security
Scenario questions that show whether the candidate can find and fix real problems methodically.
- 16
A user says they cannot reach a website. How do you troubleshoot it?
Why ask it
The core practical skill for most networking roles.What to look for
A structured approach, often bottom up: check physical connection and IP configuration, ping the gateway, ping an external IP, test DNS with nslookup or dig, try the site from another device, then check proxy, firewall or the site itself. Strong candidates scope first: one user or many, one site or all. - 17
Ping to a server works, but the application on it does not respond. What do you check?
Why ask it
Tests understanding beyond Layer 3.What to look for
Ping only proves ICMP reachability. Check whether the service is listening on the expected port, test the port with a tool such as nc or Test-NetConnection, review host and network firewalls, check TLS certificates, and read application logs. - 18
What is the difference between a stateful and a stateless firewall?
Why ask it
Firewall rules are a frequent cause of outages and security gaps.What to look for
A stateless filter checks each packet against rules (ACLs) on its own. A stateful firewall tracks connections, so return traffic for an allowed session is permitted automatically. Good answers mention least-privilege rules and documenting changes. - 19
What is the difference between latency, bandwidth and throughput?
Why ask it
Performance complaints need precise language.What to look for
Bandwidth is the maximum capacity of a link; latency is the delay for data to travel; throughput is the data actually delivered. A high-bandwidth link can still feel slow with high latency or packet loss. Candidates may mention MTU mismatches causing fragmentation or dropped packets. - 20
How does a VPN work, and when would you use a site-to-site VPN versus a remote-access VPN?
Why ask it
Common in hybrid and remote work environments.What to look for
A VPN encrypts traffic in a tunnel across an untrusted network. Site-to-site links whole networks, such as an office and a cloud network, often with IPsec; remote access connects individual users. Good answers mention authentication, split tunnelling trade-offs and zero-trust alternatives.
Running the interview well
- 1Include at least one subnetting question worked out on paper; it is quick to check and hard to bluff.
- 2Use a troubleshooting scenario and let the candidate ask questions; the questions they ask show their method.
- 3Ask for commands they would run (ping, traceroute, nslookup, ip route, show commands on a switch) and what each result would tell them.
- 4Scale depth to the role: IT support needs DHCP, DNS and basic troubleshooting; network engineers need routing protocols, VLAN design and change control.
- 5Treat certifications such as CCNA or CompTIA Network+ as useful signals, but test practical reasoning directly.
- 6Ask about an outage they helped resolve and what they changed afterwards.
Common questions
What networking topics are asked most in interviews?
The OSI and TCP/IP models, TCP versus UDP, the three-way handshake, IP addressing and subnetting, DNS, DHCP, ARP, NAT, VLANs, routing basics and troubleshooting steps.
How do I prepare for a computer networks interview?
Practise subnetting until you can do it quickly by hand, build a small lab with virtual routers and switches or a home network, and rehearse a step-by-step troubleshooting method. Be ready to explain what happens when you load a web page.
Are networking interview questions different for IT support and network engineer roles?
Yes. IT support interviews focus on DHCP, DNS, Wi-Fi, basic troubleshooting and user communication. Network engineer interviews go deeper into routing protocols, switching design, firewalls, VPNs and change management.
Do I need to memorise port numbers?
Know the common ones, such as DNS, HTTP, HTTPS, SSH and DHCP, because they come up in troubleshooting and firewall questions. You do not need to memorise long lists.
Is the OSI model still relevant?
Real networks run on TCP/IP, but the OSI model remains the common way to describe where a problem sits, such as 'a Layer 2 issue'. Interviewers use it to check your mental model.
What networking knowledge do cloud and DevOps roles need?
Subnetting and CIDR, routing tables, DNS, load balancers, firewalls and security groups, NAT, VPNs and private connectivity. Cloud networks use the same fundamentals with provider-specific names.
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