November 25, 2025
12 min read

Junior Network Engineer Interview Questions: Complete Guide

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Junior Network Engineer Interview Questions: Complete Guide
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Milad Bonakdar

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Master essential networking fundamentals with comprehensive interview questions covering TCP/IP, routing, switching, VLANs, and core network concepts for junior network engineer roles.

Introduction

Network Engineers design, implement, and maintain computer networks that enable communication and data transfer. As a junior network engineer, you'll need foundational knowledge of TCP/IP, routing, switching, and network troubleshooting.

This guide covers essential interview questions for junior network engineers, focusing on core networking concepts.

TCP/IP Fundamentals

1. Explain the OSI model and TCP/IP model.

Answer:

OSI Model (7 Layers):

  1. Physical - Cables, signals
  2. Data Link - MAC addresses, switches
  3. Network - IP addresses, routing
  4. Transport - TCP/UDP, ports
  5. Session - Connections
  6. Presentation - Encryption, formatting
  7. Application - HTTP, FTP, DNS

TCP/IP Model (4 Layers):

  1. Network Access - Physical + Data Link
  2. Internet - IP
  3. Transport - TCP/UDP
  4. Application - Application + Presentation + Session
Loading diagram...

Rarity: Very Common
Difficulty: Easy

2. What's the difference between TCP and UDP?

Answer:

FeatureTCPUDP
ConnectionConnection-orientedConnectionless
ReliabilityGuaranteed deliveryBest effort
OrderingOrderedUnordered
SpeedSlowerFaster
OverheadHigherLower
Use CasesHTTP, FTP, EmailDNS, Streaming, Gaming

TCP Three-Way Handshake:

Client          Server
  |---SYN--->      |
  |<--SYN-ACK--|   |
  |---ACK--->      |

Rarity: Very Common
Difficulty: Easy

IP Addressing

3. Explain subnetting and calculate subnet masks.

Answer: Subnetting divides a network into smaller subnetworks.

Example: 192.168.1.0/24

  • Network: 192.168.1.0
  • Subnet Mask: 255.255.255.0
  • Usable IPs: 192.168.1.1 - 192.168.1.254
  • Broadcast: 192.168.1.255

Subnetting Example:

Network: 192.168.1.0/24
Need: 4 subnets

/24 → /26 (4 subnets, 62 hosts each)

Subnet 1: 192.168.1.0/26   (192.168.1.1 - 192.168.1.62)
Subnet 2: 192.168.1.64/26  (192.168.1.65 - 192.168.1.126)
Subnet 3: 192.168.1.128/26 (192.168.1.129 - 192.168.1.190)
Subnet 4: 192.168.1.192/26 (192.168.1.193 - 192.168.1.254)

CIDR Notation:

  • /24 = 255.255.255.0 (256 addresses)
  • /25 = 255.255.255.128 (128 addresses)
  • /26 = 255.255.255.192 (64 addresses)
  • /27 = 255.255.255.224 (32 addresses)

Rarity: Very Common
Difficulty: Medium

4. Explain NAT and its types.

Answer: NAT (Network Address Translation) translates private IP addresses to public IP addresses.

Why Use NAT:

  • Conserve public IP addresses
  • Security (hide internal network)
  • Flexibility in network design

NAT Types:

1. Static NAT:

  • One-to-one mapping
  • Private IP ↔ Public IP
  • Used for servers

2. Dynamic NAT:

  • Pool of public IPs
  • First-come, first-served
  • Temporary mapping

3. PAT (Port Address Translation):

  • Many-to-one mapping
  • Uses port numbers
  • Most common (home routers)
Loading diagram...

Static NAT Configuration (Cisco):

! Configure inside interface
Router(config)# interface fastethernet 0/0
Router(config-if)# ip address 192.168.1.1 255.255.255.0
Router(config-if)# ip nat inside
Router(config-if)# exit

! Configure outside interface
Router(config)# interface fastethernet 0/1
Router(config-if)# ip address 203.0.113.5 255.255.255.0
Router(config-if)# ip nat outside
Router(config-if)# exit

! Create static NAT mapping
Router(config)# ip nat inside source static 192.168.1.10 203.0.113.10

! Verify
Router# show ip nat translations

Dynamic NAT Configuration:

! Define pool of public IPs
Router(config)# ip nat pool PUBLIC_POOL 203.0.113.10 203.0.113.20 netmask 255.255.255.0

! Define which private IPs can use NAT
Router(config)# access-list 1 permit 192.168.1.0 0.0.0.255

! Link ACL to pool
Router(config)# ip nat inside source list 1 pool PUBLIC_POOL

! Verify
Router# show ip nat translations
Router# show ip nat statistics

PAT Configuration (Overload):

! Use single public IP with port translation
Router(config)# access-list 1 permit 192.168.1.0 0.0.0.255
Router(config)# ip nat inside source list 1 interface fastethernet 0/1 overload

! Or with a pool
Router(config)# ip nat inside source list 1 pool PUBLIC_POOL overload

NAT Translation Example:

Inside Local    Inside Global    Outside Global    Outside Local
192.168.1.10    203.0.113.5      8.8.8.8          8.8.8.8
192.168.1.11    203.0.113.5      8.8.8.8          8.8.8.8

With PAT:
192.168.1.10:1234 → 203.0.113.5:50001 → 8.8.8.8:80
192.168.1.11:1234 → 203.0.113.5:50002 → 8.8.8.8:80

Troubleshooting NAT:

! Clear NAT translations
Router# clear ip nat translation *

! Debug NAT
Router# debug ip nat
Router# debug ip nat detailed

! Show NAT statistics
Router# show ip nat statistics

Limitations:

  • Breaks end-to-end connectivity
  • Complicates some protocols (FTP, SIP)
  • Not suitable for servers (use static NAT)
  • IPv6 eliminates need for NAT

Rarity: Very Common
Difficulty: Easy-Medium

Switching

4. What is a VLAN and why use it?

Answer: VLAN (Virtual LAN) logically segments a network.

Benefits:

  • Security (isolate traffic)
  • Performance (reduce broadcast domains)
  • Flexibility (group by function, not location)
  • Cost savings (fewer physical switches)

VLAN Configuration (Cisco):

! Create VLAN
Switch(config)# vlan 10
Switch(config-vlan)# name Sales
Switch(config-vlan)# exit

Switch(config)# vlan 20
Switch(config-vlan)# name IT
Switch(config-vlan)# exit

! Assign port to VLAN
Switch(config)# interface fastethernet 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10

! Configure trunk port
Switch(config)# interface gigabitethernet 0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20

! Verify
Switch# show vlan brief
Switch# show interfaces trunk

Rarity: Very Common
Difficulty: Medium

6. What is Spanning Tree Protocol and why is it needed?

Answer: STP (Spanning Tree Protocol) prevents Layer 2 loops in switched networks.

Problem Without STP:

  • Broadcast storms
  • MAC table instability
  • Multiple frame copies
  • Network meltdown

How STP Works:

Loading diagram...

STP Port States:

  1. Blocking: Doesn't forward frames, prevents loops
  2. Listening: Preparing to forward, listening for BPDUs
  3. Learning: Learning MAC addresses
  4. Forwarding: Normal operation
  5. Disabled: Administratively down

Port Roles:

  • Root Port: Best path to root bridge
  • Designated Port: Forwarding port on segment
  • Blocked Port: Prevents loops

STP Selection Process:

1. Elect Root Bridge (lowest Bridge ID)
   Bridge ID = Priority (default 32768) + MAC Address

2. Select Root Ports (best path to root)
   Based on: Path Cost → Bridge ID → Port ID

3. Select Designated Ports (one per segment)

4. Block remaining ports

STP Configuration (Cisco):

! View STP status
Switch# show spanning-tree

! Set bridge priority (make this switch root)
Switch(config)# spanning-tree vlan 1 priority 4096
# Priority must be multiple of 4096 (0-61440)

! Or use shortcut
Switch(config)# spanning-tree vlan 1 root primary
Switch(config)# spanning-tree vlan 1 root secondary

! Configure port cost
Switch(config)# interface gigabitethernet 0/1
Switch(config-if)# spanning-tree cost 4

! Configure port priority
Switch(config-if)# spanning-tree port-priority 64

! Enable PortFast (for end devices only!)
Switch(config-if)# spanning-tree portfast

! Enable BPDU Guard (shuts port if BPDU received)
Switch(config-if)# spanning-tree bpduguard enable

STP Variants:

ProtocolStandardConvergenceVLANs
STP802.1D50 secondsAll
RSTP802.1w~6 secondsAll
PVST+Cisco50 secondsPer-VLAN
Rapid PVST+Cisco~6 secondsPer-VLAN

RSTP (Rapid Spanning Tree):

! Enable RSTP
Switch(config)# spanning-tree mode rapid-pvst

! Verify
Switch# show spanning-tree summary

RSTP Port States (Simplified):

  • Discarding: Combines Blocking, Listening, Disabled
  • Learning: Learning MAC addresses
  • Forwarding: Normal operation

Troubleshooting STP:

! Check STP status
Switch# show spanning-tree

! Check specific VLAN
Switch# show spanning-tree vlan 10

! Check interface details
Switch# show spanning-tree interface gigabitethernet 0/1

! View root bridge
Switch# show spanning-tree root

! Debug STP
Switch# debug spanning-tree events

Common Issues:

  1. Topology Changes:

    • Frequent changes cause instability
    • Use PortFast on access ports

  2. Root Bridge Placement:

    • Should be central, high-capacity switch
    • Set priority manually

  3. Loops:

    • Enable BPDU Guard on access ports
    • Monitor for unexpected topology changes

Rarity: Common
Difficulty: Medium

Routing

7. What's the difference between static and dynamic routing?

Answer:

Static Routing:

  • Manually configured
  • No overhead
  • Doesn't adapt to changes
  • Good for small, stable networks

Dynamic Routing:

  • Automatically learns routes
  • Adapts to topology changes
  • More overhead
  • Good for large, complex networks

Static Route Example:

! Add static route
Router(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.1

! Default route
Router(config)# ip route 0.0.0.0 0.0.0.0 10.0.0.1

! Verify
Router# show ip route

Dynamic Routing Protocols:

  • RIP: Simple, distance-vector
  • OSPF: Link-state, fast convergence
  • EIGRP: Cisco proprietary, hybrid
  • BGP: Internet routing

Rarity: Very Common
Difficulty: Easy-Medium

8. How do you configure Access Control Lists (ACLs)?

Answer: ACLs filter network traffic based on defined rules.

ACL Types:

1. Standard ACL (1-99, 1300-1999):

  • Filters based on source IP only
  • Applied close to destination

2. Extended ACL (100-199, 2000-2699):

  • Filters based on source/dest IP, protocol, port
  • Applied close to source

Standard ACL Example:

! Create standard ACL
Router(config)# access-list 10 permit 192.168.1.0 0.0.0.255
Router(config)# access-list 10 deny any

! Apply to interface
Router(config)# interface fastethernet 0/0
Router(config-if)# ip access-group 10 in

! Verify
Router# show access-lists
Router# show ip interface fastethernet 0/0

Extended ACL Example:

! Create extended ACL
Router(config)# access-list 100 permit tcp 192.168.1.0 0.0.0.255 any eq 80
Router(config)# access-list 100 permit tcp 192.168.1.0 0.0.0.255 any eq 443
Router(config)# access-list 100 deny ip any any

! Apply to interface
Router(config)# interface fastethernet 0/1
Router(config-if)# ip access-group 100 out

Named ACL (Recommended):

! Standard named ACL
Router(config)# ip access-list standard ALLOW_INTERNAL
Router(config-std-nacl)# permit 192.168.1.0 0.0.0.255
Router(config-std-nacl)# deny any
Router(config-std-nacl)# exit

! Extended named ACL
Router(config)# ip access-list extended WEB_TRAFFIC
Router(config-ext-nacl)# permit tcp any any eq 80
Router(config-ext-nacl)# permit tcp any any eq 443
Router(config-ext-nacl)# permit icmp any any echo-reply
Router(config-ext-nacl)# deny ip any any
Router(config-ext-nacl)# exit

! Apply to interface
Router(config)# interface gigabitethernet 0/0
Router(config-if)# ip access-group WEB_TRAFFIC in

Wildcard Masks:

0 = must match
1 = don't care

Examples:
0.0.0.0 = exact match
0.0.0.255 = match first 3 octets (/24)
0.0.255.255 = match first 2 octets (/16)
255.255.255.255 = match any (same as "any")

Host:
192.168.1.10 0.0.0.0 = single host
Or use: host 192.168.1.10

Common ACL Scenarios:

1. Block specific host:

Router(config)# ip access-list extended BLOCK_HOST
Router(config-ext-nacl)# deny ip host 192.168.1.50 any
Router(config-ext-nacl)# permit ip any any

2. Allow only SSH and HTTPS:

Router(config)# ip access-list extended SECURE_ACCESS
Router(config-ext-nacl)# permit tcp any any eq 22
Router(config-ext-nacl)# permit tcp any any eq 443
Router(config-ext-nacl)# deny ip any any

3. Prevent spoofing:

Router(config)# ip access-list extended ANTI_SPOOF
Router(config-ext-nacl)# deny ip 192.168.1.0 0.0.0.255 any
Router(config-ext-nacl)# deny ip 10.0.0.0 0.255.255.255 any
Router(config-ext-nacl)# permit ip any any

ACL Best Practices:

  1. Order Matters:

    • Processed top to bottom
    • Most specific rules first
    • Implicit deny at end

  2. Placement:

    • Standard ACL: Close to destination
    • Extended ACL: Close to source

  3. Documentation:

    • Use named ACLs
    • Add remarks
Router(config)# ip access-list extended FIREWALL
Router(config-ext-nacl)# remark Allow web traffic
Router(config-ext-nacl)# permit tcp any any eq 80
Router(config-ext-nacl)# remark Block internal network
Router(config-ext-nacl)# deny ip 192.168.0.0 0.0.255.255 any

Editing ACLs:

! View ACL with line numbers
Router# show ip access-lists WEB_TRAFFIC

! Remove specific line
Router(config)# ip access-list extended WEB_TRAFFIC
Router(config-ext-nacl)# no 10

! Insert at specific line
Router(config-ext-nacl)# 15 permit tcp any any eq 8080

Troubleshooting:

! Show ACL hits
Router# show access-lists

! Show ACL on interface
Router# show ip interface gigabitethernet 0/0

! Clear ACL counters
Router# clear access-list counters

Rarity: Common
Difficulty: Medium

Network Services

9. How does DHCP work?

Answer: DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses.

DORA Process:

  1. Discover: Client broadcasts request
  2. Offer: Server offers IP address
  3. Request: Client requests offered IP
  4. Acknowledge: Server confirms assignment
Loading diagram...

DHCP Configuration (Cisco):

! Configure DHCP pool
Router(config)# ip dhcp pool LAN
Router(dhcp-config)# network 192.168.1.0 255.255.255.0
Router(dhcp-config)# default-router 192.168.1.1
Router(dhcp-config)# dns-server 8.8.8.8 8.8.4.4
Router(dhcp-config)# lease 7

! Exclude addresses
Router(config)# ip dhcp excluded-address 192.168.1.1 192.168.1.10

! Verify
Router# show ip dhcp binding
Router# show ip dhcp pool

Rarity: Very Common
Difficulty: Easy-Medium

Troubleshooting

10. How do you troubleshoot network connectivity issues?

Answer: Systematic troubleshooting approach:

1. Verify Physical Layer:

# Check cable connection
# Check link lights
# Check port status

2. Test Connectivity:

# Ping localhost
ping 127.0.0.1

# Ping default gateway
ping 192.168.1.1

# Ping external IP
ping 8.8.8.8

# Ping domain name
ping google.com

3. Check IP Configuration:

# Windows
ipconfig /all

# Linux
ip addr show
ip route show

# Verify:
# - IP address
# - Subnet mask
# - Default gateway
# - DNS servers

4. Test DNS:

# Windows
nslookup google.com

# Linux
dig google.com
host google.com

5. Check Routing:

# Trace route
traceroute google.com  # Linux
tracert google.com     # Windows

6. Check Firewall:

# Test specific port
telnet server.com 80
nc -zv server.com 80

Rarity: Very Common
Difficulty: Medium

Conclusion

Preparing for a junior network engineer interview requires understanding networking fundamentals and hands-on practice. Focus on:

  1. TCP/IP: OSI model, protocols, addressing
  2. IP Addressing: Subnetting, CIDR, IPv4/IPv6
  3. NAT: Types, configuration, use cases
  4. Switching: VLANs, trunking, MAC addresses
  5. STP: Loop prevention, port states, RSTP
  6. Routing: Static vs dynamic, routing tables
  7. ACLs: Standard vs extended, wildcard masks
  8. Network Services: DHCP, DNS, NAT
  9. Troubleshooting: Systematic approach, tools

Practice with network simulators (Packet Tracer, GNS3) and real equipment. Good luck!

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