These CCNA interview questions cover the networking fundamentals that entry-level and junior network engineer interviews focus on. They follow the main topics of the Cisco CCNA exam: network fundamentals, switching, routing, IP services and security. Each answer is short, with links to deeper guides on this blog.
Network fundamentals
1. Explain the OSI model in one sentence per layer.
Physical sends bits, Data Link moves frames between neighbours using MAC addresses, Network routes packets with IP addresses, Transport delivers segments end to end (TCP or UDP), Session manages conversations, Presentation formats and encrypts data, and Application provides network services to programs. See our OSI model analogy.
2. TCP vs UDP?
TCP is connection-oriented and reliable, with a three-way handshake, acknowledgements and retransmission. UDP is connectionless and faster, but it does not guarantee delivery. Web traffic uses TCP, while DNS queries, VoIP and video streams often use UDP.
3. Describe the TCP three-way handshake.
The client sends SYN, the server replies SYN-ACK, and the client answers ACK. Then data can flow.
4. Hub vs switch vs router?
A hub repeats every signal to all ports. A switch forwards frames only to the right port using a MAC address table. A router connects different networks and forwards packets based on IP addresses.
5. Collision domain vs broadcast domain?
Each switch port is its own collision domain. A whole VLAN is one broadcast domain, and routers separate broadcast domains.
IP addressing and subnetting
6. How many usable hosts are in a /26 network?
A /26 leaves 6 host bits: 2⁶ = 64 addresses, minus the network and broadcast addresses, gives 62 usable hosts.
7. What are the private IPv4 ranges?
10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, defined in RFC 1918.
8. Name the main IPv6 address types.
Global unicast (public), unique local (private, fc00::/7), link-local (fe80::/10) and multicast. IPv6 has no broadcast. Read more in IPv4 vs IPv6.
Switching
9. What is a VLAN, and why use one?
A VLAN splits a switch into separate logical networks. It improves security and reduces broadcast traffic. See how to configure VLANs.
10. Access port vs trunk port?
An access port carries one VLAN to an end device. A trunk carries many VLANs between switches, using 802.1Q tags.
11. Why do we need Spanning Tree Protocol?
Redundant switch links create loops, which cause broadcast storms. STP blocks some ports to keep one loop-free path, and unblocks them if the active path fails. Rapid PVST+ and RSTP converge much faster than the original 802.1D.
12. What is EtherChannel?
EtherChannel bundles several physical links into one logical link for more bandwidth and redundancy, negotiated with LACP or PAgP.
Routing
13. Static vs dynamic routing?
Static routes are entered manually. Dynamic protocols, such as OSPF, learn and update routes automatically. See types of routing.
14. What is administrative distance?
It ranks the trustworthiness of route sources; lower wins. For example, a static route is 1 and OSPF is 110. Read more about administrative distance.
15. How does OSPF choose the best path?
It uses cost, based on interface bandwidth, and runs Dijkstra’s algorithm. See our OSPF guide.
16. What is a default route?
ip route 0.0.0.0 0.0.0.0 next-hop sends traffic for unknown destinations to one next hop, usually the ISP.
IP services and security
17. What does NAT do?
NAT translates private addresses to public ones. PAT (NAT overload) lets many devices share one public IP by using different port numbers.
18. How does DHCP work?
Clients get addresses through Discover, Offer, Request and Acknowledge. Routers forward requests to remote servers with ip helper-address. See how to configure DHCP.
19. Standard vs extended ACL?
Standard ACLs match only the source address. Extended ACLs match source, destination, protocol and port. See types of ACLs.
20. How do you secure a switch?
Disable unused ports, use port security or 802.1X, enable DHCP snooping and Dynamic ARP Inspection, use SSH instead of Telnet, and centralise logins with AAA.
21. Name the main Wi-Fi standards.
Wi-Fi 5 (802.11ac), Wi-Fi 6 and 6E (802.11ax) and Wi-Fi 7 (802.11be). For security, use WPA3 or WPA2-Enterprise.
Keep preparing
Practise each answer out loud, and run the commands in a lab rather than memorising them. Interviewers often ask follow-up questions, so make sure you understand why each answer is true.
Sources
Preparing for an interview? Browse more questions in our interview preparation hub. For businesses, we also offer security services such as VAPT, server hardening and ISO 27001 readiness.
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