Networking

What interviewers look for

• A layered troubleshooting method: cable and link lights first, then IP address, gateway and DNS, and only then the application.
• Subnetting you can do on paper: how many hosts fit, which addresses are usable, and whether two devices are on the same network.
• Hands-on work with the kit you will really touch: MikroTik or Cisco routers, managed switches and access points.
• Documentation and change habits: a network diagram, an IP address list, labelled cables, and a saved config before every change.
• Security sense: separate guest Wi-Fi, no default passwords on any device, and firmware kept up to date.
• Calm communication during an outage: short, plain updates to managers about what is affected and when you expect it back.

Questions they ask

1. 1“Explain the OSI model, and how you use it when you troubleshoot.”

   Why they ask: They do not want the seven layers recited. They want to see you use the layers as a checklist to find where a fault is.

   How to answer

   • Name the layers you use most: physical, data link, network, transport and application.
   • Explain that you work from the bottom up, so you never guess.
   • Give one real fault and the layer where you found it.

   I use the layers as a checklist, from the bottom. Once a finance laptop could not reach the accounting server. Layer 1: the link light on the switch port was on. Layers 2 and 3: the laptop had a correct IP address but could not ping the gateway, so I checked the switch and found the port had been moved to the guest VLAN. I put it back in the staff VLAN, the ping worked, and the application opened. Working upwards stops me from guessing.

2. 2“You have 192.168.10.0/24. Staff are 60 users, and you also need guests and printers. How would you split it?”

   Why they ask: It is subnetting in real life: counting hosts, choosing masks, leaving room to grow, and writing a clear plan.

   How to answer

   • Count the devices in each group and add room to grow before you pick a mask.
   • Give each group its own subnet and VLAN, with the gateway on the first usable address.
   • Write the plan in a table and keep one block spare.

   First I count and add room to grow. Staff are 60 now, so I give them a /25: 192.168.10.0 to .127, which is 126 usable addresses. Guests get a /26, 192.168.10.128 to .191. Printers and servers get a /27, .192 to .223, with fixed addresses. That leaves .224/27 spare. Each subnet is its own VLAN, the gateway is the first usable address, and I write the plan in a table before I configure anything.

3. 3“What is the difference between a switch and a router?”

   Why they ask: A basic check that you know which device does what, and where each one sits in a real office network.

   How to answer

   • A switch connects devices inside one network and forwards frames by MAC address (layer 2).
   • A router connects different networks by IP address (layer 3), for example the office and the internet, often with NAT and DHCP.
   • Place them in an example: switches on each floor, the router at the edge towards the provider.

4. 4“A user's computer shows an IP address starting with 169.254. What does it mean, and what do you check?”

   Why they ask: It tests whether you understand DHCP and can narrow the fault down: one device, one switch port, or the DHCP server itself.

   How to answer

   • Say it means the computer got no answer from a DHCP server and gave itself an address.
   • One user: check the cable, the switch port and its VLAN.
   • Many users: check the DHCP server is running and its address pool is not full.

   An address starting with 169.254 means the computer asked for an address and no DHCP server answered, so it gave itself one. First I check whether it is one user or many. If it is one, I look at the cable and the switch port, including which VLAN the port is in. If it is many, I check the DHCP server: is it running, and is the pool full? At one office the guest pool was full of old leases from visitors' phones, so I shortened the lease time on that network.

5. 5“What is a VLAN, and why would an office use one?”

   Why they ask: VLANs are daily work in most offices. They check you understand the idea and the difference between access and trunk ports.

   How to answer

   • A VLAN splits one physical switch into separate networks, such as staff, guests and cameras.
   • Benefits: security, less broadcast traffic, and simpler rules per group.
   • Access ports carry one VLAN to a device; trunk ports carry tagged VLANs between switches and to the router.

6. 6“Tell us about a network you set up or looked after. What would you do differently now?”

   Why they ask: They check real experience: its size, your exact part, a real problem, and whether you learned from it.

   How to answer

   • Give the size and the equipment: users, buildings, router and switch models.
   • Say exactly what you did and what someone else did.
   • End with one lesson, such as documentation or backups.

   At a private university in Hargeisa I supported the IT officer on a network of about 120 computers in two buildings joined by fibre. My part was the switches and the MikroTik router: I set up separate VLANs for staff, students and the labs, and a guest Wi-Fi with a speed limit. The main problem was students getting the staff Wi-Fi password, so we changed it every term. Now I would document from day one; we drew the network diagram only after the first outage.

7. 7“How do you make a change on a live router without taking the office offline?”

   Why they ask: One wrong command can cut off a whole bank branch. They want to hear backups, timing, a way back and testing.

   How to answer

   • Save or export the current config first, and plan the change outside busy hours.
   • Use a way back: MikroTik Safe Mode, or a timed reload on Cisco before saving.
   • Test, then save and write the change in the log so the team knows.

Example answers are in English, the language most panels use. Say it in your own words.

Topics to revise

• OSI and TCP/IP modelsSeven OSI layers: physical, data link, network, transport, session, presentation, application. TCP/IP groups them into four. Know one example for each key layer: cable (1), switch and MAC address (2), router and IP (3), TCP and UDP ports (4), HTTP and DNS (7).
• Subnetting (/24, /25, /26, /27, /30)The number after the slash is how many bits are the network. Usable hosts = 2 to the power of (32 minus the prefix), minus 2. So /24 = 254, /25 = 126, /26 = 62, /27 = 30, /30 = 2 for a link between two routers. A classic question: for 192.168.1.70/26, the network is .64 and the broadcast is .127.
• Switch, router and access pointA switch joins devices inside one network using MAC addresses. A router joins networks using IP addresses and sends traffic to the internet. An access point gives Wi-Fi to devices and connects them to the wired network. A layer 3 switch can also route between VLANs.
• DHCPDHCP hands out the IP address, mask, gateway and DNS automatically. The four steps are Discover, Offer, Request, Acknowledge (DORA). Know lease time, reservations for printers and servers, and that an address starting 169.254 means DHCP did not answer.
• DNSDNS turns names like example.com into IP addresses. To tell a DNS fault from a connection fault: if ping 8.8.8.8 works but a website will not open by name, it is DNS. Use nslookup to test. They may ask what DNS server your office should use and why.
• VLANs, access ports and trunk portsA VLAN is a separate network on shared switches. An access port belongs to one VLAN and connects a computer or printer. A trunk port carries many VLANs, each with an 802.1Q tag, between switches or to the router. Traffic between VLANs needs a router or a layer 3 switch.
• Private IP ranges and NATPrivate ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16; they are not reachable from the internet. NAT lets many private devices share one public IP. Port forwarding opens one service, such as a camera recorder, to the outside, and must be done carefully.
• TCP vs UDP and common portsTCP checks that data arrives in order (web, email, file transfer). UDP is faster with no guarantee (voice calls, video, most DNS). Ports to know: 22 SSH, 53 DNS, 80 HTTP, 443 HTTPS, 3389 Remote Desktop, 8291 MikroTik Winbox.
• Troubleshooting commandsipconfig /all shows the address, gateway and DNS on Windows. ping tests if a device answers. tracert (traceroute) shows where along the path traffic stops. nslookup tests DNS. arp -a shows which MAC address is behind an IP. On MikroTik, Torch shows live traffic on an interface.
• Wi-Fi: 2.4 GHz vs 5 GHz, channels, WPA2/WPA32.4 GHz reaches further through walls but is slower and crowded; use channels 1, 6 or 11 so neighbours do not overlap. 5 GHz is faster with a shorter range. Use WPA2 or WPA3, a separate guest network, and place access points where people actually sit.
• MikroTik and Cisco basicsMikroTik: Winbox or WebFig, IP addresses, DHCP server, a NAT masquerade rule for internet, Queues for speed limits, /export to back up the config, and Safe Mode for risky changes. Cisco IOS: enable, configure terminal, show ip interface brief, show running-config, and copy running-config startup-config to save.

Practical tasks you may get

1. 1Live troubleshooting on real kit: a laptop, a switch and a router with one fault planted, such as a wrong gateway, a port in the wrong VLAN, DHCP switched off or a bad cable. Say each layer out loud as you check it. Practise by breaking and fixing your own small lab at home.
2. 2Subnetting on paper in 20 to 30 minutes: a network and the size of each department; you write the subnets, masks, gateways and ranges. Practise until you can do /24 to /30 without a calculator.
3. 3Configure a router from scratch: an IP address, a DHCP server, NAT to the internet, and a guest network with a speed limit, on MikroTik or Cisco. Practise on an old router or in a simulator such as Cisco Packet Tracer.
4. 4Draw the network on a whiteboard: a small office or two branches, with the provider link, router, firewall, switches, VLANs, access points and server. Practise drawing one network you know, with its IP ranges.

Mistakes to avoid

• Reciting the seven OSI layers but not being able to use them to find a fault.
• Restarting the router for every problem without ever finding the cause.
• Changing a live router during working hours with no saved config and no way back.
• Leaving default admin passwords on routers, cameras and access points.
• Keeping IP addresses and passwords only in your head, with no diagram or list.
• Talking in technical words to a manager during an outage instead of saying what is affected and when it will be back.

Quick check

5 questions. Answer each one to see the explanation.