Network Admin Guide Tutorial

This document includes network issues across OSI Layers (L1–L7) with explanations and solutions supported by simulations.

1

IP Conflict (IP Collision)

Network Layer (L3)
Problem Definition: Two devices on the network (for example, two PCs) are mistakenly configured with the same IP address (for example, 192.168.1.10). In this case, the devices conflict on the network and cannot communicate.

Symptoms:

  • In some cases, "Duplicate IP Address Detected" warning may be visible
  • Network connection is lost or becomes unstable
PC0 IP Configuration

PC0: IP configuration setup

PC1 Same IP - Conflict

PC1: Same IP address causing conflict

PC1 Different IP - Fixed

PC1: Different IP address assigned

Ping Successful

Success: PC1 to PC0 ping successful after IP change

Solution
  • Assign a unique IP address to each device
  • IP addresses should be distributed in a planned manner or assigned automatically using DHCP
  • Verify that each device has a different IP address in the same subnet

Result: IP conflicts are Layer 3 (Network Layer) problems that prevent proper routing and communication between devices. With unique IP assignment, network communication is restored and devices can communicate successfully.

Download Simulation
2

MAC Address Conflict (MAC Address Collision)

Data Link Layer (L2)
Problem Definition: Two computers in the network (for example PC0 and PC1) are mistakenly configured with the same MAC address. The switch learns MAC addresses according to ports, but when the same MAC address comes from multiple ports, routing instability occurs. This situation leads to packet loss, ping failures, and general network instability.

Symptoms:

  • Ping command yields no results (Request timed out)
  • Network performance decreases
  • ARP tables get confused
PC0 IP Configuration

PC0: IP configuration (192.168.1.1)

PC1 IP Configuration

PC1: IP configuration (192.168.1.2)

Same MAC Address Problem

PC0's MAC address copied to PC1 (Problem created)

Ping Failed

Ping from PC1 to PC0: Request timed out

MAC Address Fixed

PC1: MAC address changed to unique value

Ping Successful

Success: Ping successful after MAC address fix

Solution

Each device must have a unique MAC address.

Solution Steps:

  1. Go to PC1 → Config > FastEthernet
  2. Change the MAC address (for example, enter a different value like 00D0.BA1D.1C02)
  3. Repeat the ping test

Result:

  • Ping becomes successful
  • Switch MAC table works properly
  • Network instability is eliminated

Technical Note: This is a Layer 2 (Data Link Layer) problem. MAC addresses are used for frame switching within the local network, and conflicts prevent proper frame delivery.

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3

VLAN Misconfiguration (VLAN Wrong Configuration)

Data Link Layer (L2)
Problem Definition: Two devices in the network (for example, computers belonging to different departments) should be assigned to different VLANs, but are mistakenly configured in the same VLAN. Since these devices have different IP subnets, communication fails when they try to communicate within the same VLAN.
PC0 IP Configuration

PC0: IP configuration (192.168.10.10)

PC1 IP Configuration

PC1: IP configuration (192.168.20.10)

VLAN Misconfiguration - Failed

Ping failed due to VLAN misconfiguration

VLAN Configuration - CLI Commands

Switch CLI: VLAN configuration commands

Solution

🔧 Step 1: Create Correct VLANs

Switch(config)# vlan 10
Switch(config)# vlan 20

🔧 Step 2: Assign Ports to Appropriate VLANs

Switch(config)# interface fa0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10

Switch(config)# interface fa0/2
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 20

🔧 Step 3: Add Routing If Needed

If communication between VLAN 10 and VLAN 20 is desired, Router-on-a-Stick or Layer 3 Switch routing configuration is required.

Result: The main cause of this problem is incorrect configuration of logical separation (VLAN/subnet) despite correct physical connections. Although this problem appears at Layer 2 level, it also has Layer 3 effects. With proper VLAN assignment, network segmentation is achieved and communication is re-established.

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4

Cable Fault

Physical Layer (L1)
Description: A physical cable was disconnected between PC and switch, causing 100% packet loss.
Cable Fault - Disconnected

Cable disconnected: Ping fails

Cable Fault - Fixed

Cable reconnected: Ping succeeds

Solution

Diagnose the physical connection and restore the faulty cable. Use cable testers to verify cable integrity. Check link lights to confirm connection status.

Download Simulation
5

Default Gateway Missing

Network Layer (L3)
Description: Even though the computer (PC) is configured with the correct IP address and subnet mask, if the default gateway information is not entered, it cannot communicate with devices outside its own subnet. The default gateway is the router address that a device refers to when accessing other networks outside its own network. When this information is missing, packets do not know where to be sent and are lost.
PC1 IP Settings

PC1: 192.168.1.2/24, Gateway: 192.168.1.1 (Correct)

PC2 Gateway Missing

PC2: 192.168.2.2/24, Gateway: Empty (Problem)

Router Interface 1

Router Interface Giga0/0: 192.168.1.1

Router Interface 2

Router Interface Giga0/1: 192.168.2.1

Ping Failed

Ping fails: Request timed out due to missing gateway

PC2 Gateway Fixed

PC2 Gateway fixed: 192.168.2.1 configured

Ping Success

Ping successful after fixing gateway configuration

Solution

Configure Default Gateway: PC2 must be configured with the correct Default Gateway address (192.168.2.1 - the router's interface on PC2's side). Once configured:

  • PC2 correctly forwards packets to the router
  • Router forwards packets to the other network
  • Ping operation completes successfully
  • Inter-VLAN routing works properly

Technical Note: This is a Network Layer (L3) issue. IP routing fails because the exit point (router IP) is not defined for routing decisions.

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6

DNS Server Unreachable

Application Layer (L7)
Description: When a user wants to access a website and types the domain name (www.gokay.com) in the address bar, the page does not open. However, if access can be provided with the IP address, this indicates that there is no problem in the network layers; but the domain name cannot be translated to an IP address. This occurs due to the inability to access the DNS (Domain Name System) server and is considered an Application Layer (Layer 7) problem.
Common Causes:
  • Client device's DNS address may be incorrectly defined
  • DNS server is not configured or the service is not active
  • DNS record (domain name → IP mapping) is missing
  • No connection between server and client
Server IP Configuration

Server IP Configuration: 192.168.1.1 configured

DNS Server Configuration

DNS Service Configuration: www.gokay.com → 192.168.1.1

PC0 Wrong DNS Config

PC0: Wrong DNS server address configured (Problem)

DNS Resolution Failed

www.gokay.com: Host name unresolved due to wrong DNS

PC0 DNS Fixed

PC0: DNS server address corrected to 192.168.1.1

DNS Resolution Success

Success: www.gokay.com accessed after fixing PC0 DNS

Solution

To solve the problem, the following basic points should be checked:

  1. The client device's DNS address should correctly match the DNS server's IP address
  2. DNS server must be configured and active
  3. A record for the desired domain name (e.g., www.gokay.com → 192.168.1.1) must be created on the DNS server
  4. If HTTP service is active on the server, the website will be accessible after DNS resolution

Result: With proper configurations, when the user enters a domain name, this name is translated to an IP address and the page opens successfully.

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7

IP Routing Missing

Network Layer (L3)
Description: Two computers not on the same local network (in different subnets) cannot communicate when they ping each other. Although each computer works successfully on its own network, when it sends a packet to a computer from another network, it receives "Destination Host Unreachable" or "Request Timed Out" error. This situation usually occurs when routing is not done between routers. Even if routers have established connections, data packets cannot be transmitted if inter-network route instructions are not made.
Example Network Structure:
  • PC0: 192.168.1.2/24 — connected router: Router0
  • PC1: 192.168.2.2/24 — connected router: Router1
  • Inter-router link: 10.0.0.0/24 network between Router0 ↔ Router1
  • Problem: Routers connected, IPs assigned but routing not configured
PC0 IP Configuration

PC0: 192.168.1.2/24 - Gateway: 192.168.1.1

PC1 IP Configuration

PC1: 192.168.2.2/24 - Gateway: 192.168.2.1

Router0 Giga0 Config

Router0 Giga0/0: 192.168.1.1 (PC0 side)

Router0 Giga1 Config

Router0 Giga0/1: 10.0.0.1 (Inter-router link)

Router1 Giga0 Config

Router1 Giga0/0: 10.0.0.2 (Inter-router link)

Router1 Giga1 Config

Router1 Giga0/1: 192.168.2.1 (PC1 side)

Ping Failed

PC0 → PC1: Destination Host Unreachable (No Routing)

Router0 CLI Config

Router0 CLI: Static route to 192.168.2.0 via 10.0.0.2

Router1 CLI Config

Router1 CLI: Static route to 192.168.1.0 via 10.0.0.1

Ping Success

Success: PC0 → PC1 ping successful after routing config

Solution

Static routing configuration should be applied to both routers to define routes to opposite networks:

Router0 Configuration:

ip route 192.168.2.0 255.255.255.0 10.0.0.2

"To reach 192.168.2.0 network, use 10.0.0.2 (Router1) as next hop"

Router1 Configuration:

ip route 192.168.1.0 255.255.255.0 10.0.0.1

"To reach 192.168.1.0 network, use 10.0.0.1 (Router0) as next hop"

Result: After routing definitions are made, PC0 can ping PC1, two different networks communicate smoothly, and data packets find the correct route through IP routing.

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8

DHCP Gateway Misconfiguration

Network Layer (L3)
Description: Client devices on the network (e.g., PC0 and PC1) automatically obtain their IP addresses via DHCP. However, when they try to access the internet or other networks, they receive "Request Timed Out" errors. Although IP addresses appear correct, connection cannot be established. This occurs when the Default Gateway in the DHCP server configuration is incorrectly entered.
Technical Cause: In the DHCP Pool configuration, an incorrect Default Gateway was configured (e.g., 192.168.10.1 - a non-existent address). Clients receive:
  • IP Address: 192.168.1.x ✅
  • Subnet Mask: 255.255.255.0 ✅
  • Default Gateway: 192.168.10.1 ❌ (Unreachable IP!)

Therefore, clients can only access their own subnet but cannot reach the router and internet connection.

Server Static IP Config

Server FastEthernet: Static IP 192.168.1.1 configured

DHCP Wrong Gateway Config

DHCP Pool1: Wrong Default Gateway 192.168.10.1 (Problem)

PC0 DHCP Config

PC0: DHCP selected, receiving automatic IP address

PC1 DHCP Config

PC1: DHCP selected, receiving automatic IP address

Ping Failed Wrong Gateway

PC0: Ping to 192.168.10.1 timed out (Wrong gateway)

DHCP Gateway Fixed

DHCP Pool1: Default Gateway corrected to 192.168.1.1

Ping Success

Success: PC0 ping to 192.168.1.1 successful after fix

Solution Steps

1. DHCP Server Configuration Fix:

  • Go to Server0 → Services → DHCP section
  • Check the existing Pool (e.g., Pool1) Default Gateway value
  • Change incorrect IP (192.168.10.1) to correct gateway: 192.168.1.1
  • Save settings with Save button

2. Client IP Renewal:

On PC0 or PC1 → Desktop → Command Prompt, enter these commands:

ipconfig /release
ipconfig /renew

These commands make the client release current IP and obtain new (correct gateway) DHCP configuration.

3. Connection Test:

After receiving new IP information, ping the router:

ping 192.168.1.1

✅ Now you will receive successful response instead of "Request Timed Out" error.

Result: This problem is caused by the incorrect Default Gateway address distributed from the DHCP server to clients. When the gateway address is correctly updated and clients renew their IP information, the connection problem is resolved.

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9

Switching Loop (STP Disabled)

Data Link Layer (L2)
Description: At Layer 2 (Data Link Layer) level, when multiple switches are connected with redundant links, a switching loop can occur. This situation is especially seen when Spanning Tree Protocol (STP) is disabled or not configured. Switching loops cause problems such as broadcast storms (unlimited multiplying broadcast packets), slowing network traffic, and disconnection of communication between devices.
Network Scenario: In Cisco Packet Tracer, this scenario was set up:
  • Topology: 3 Switches (Switch0, Switch1, Switch2) connected in triangular formation
  • Endpoints: 2 PCs (PC0 and PC1) at the edge points
  • Normal operation: One connection being orange (blocking) shows STP preventing loops
  • Problem: When STP is disabled, all connections become active and network loop begins
PC0 IP Configuration

PC0: 192.168.1.2/24 - Gateway: 192.168.1.1

PC1 IP Configuration

PC1: 192.168.1.3/24 - Gateway: 192.168.1.1

STP Disabled

Switch: STP disabled - All ports active (Loop created)

Switching Loop Demo

PC0 ping timeout due to switching loop

🎥 Video: PC0 ping timed out for very long time (Loop effect)

STP Enabled Commands

Switch CLI: STP enabled with spanning-tree vlan 1 command

Ping Success

Success: PC0 to PC1 ping successful after STP enabled

Solution: Enable Spanning Tree Protocol

⚠️ Observed Symptoms:

  • Ping requests in the network are delayed or unreachable
  • Connection between PCs occasionally drops
  • Unexpected slowdowns in network traffic
  • Broadcast storm effects

🔧 STP Configuration Steps:

Access each switch via CLI (command line) and enter:

Switch> enable
Switch# configure terminal
Switch(config)# spanning-tree vlan 1

This command manually enables STP on VLAN 1. When STP is active, one of the ports that could create a loop goes into blocking (orange) mode and prevents the loop.

Result: Spanning Tree Protocol (STP) provides network security and continuity at Layer 2 level. If STP is enabled, loops are automatically prevented in topologies created with multiple connections. If STP is disabled, switching loops can occur and this causes serious performance and security issues in real networks.

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10

MTU Mismatch (Maximum Transmission Unit)

Data Link (L2) + Network Layer (L3)
Description: Different MTU (Maximum Transmission Unit) values are configured between two routers or between a PC and router in the network. This causes fragmentation or complete communication failure when large data packets are transmitted.
Example Scenario:
  • Router A interface MTU: 1500 bytes
  • Router B interface MTU: 1400 bytes
  • Problem: If a device sends large packets with Don't Fragment (DF) bit set, packets cannot reach the destination

How to Observe the Problem:

  • Ping command may work, but connection fails with large packets
  • Testing with ping -l 1472 -f 192.168.1.1 returns "Packet needs to be fragmented but DF set" error

Real-World Examples:

  • VPN tunnels
  • MPLS or GRE tunnels
  • ISP routing MTU limitations (e.g., 1492 or 1472)
Solution Options

🔧 Option 1: MTU Synchronization

Set the same MTU value (e.g., 1400) on all relevant interfaces:

Router(config)# interface GigabitEthernet0/0
Router(config-if)# mtu 1400

⚠️ Option 2: Disable Path MTU Discovery (Not Recommended)

Allow the device to fragment large packets (not secure, not recommended):

ping -l 1472 -f 192.168.1.1 ❌ (DF bit set)
ping -l 1472 192.168.1.1 ✅ (Fragmentation allowed)

🔧 Option 3: TCP MSS (Maximum Segment Size) Adjustment

Inform TCP-connecting devices about maximum segment size via router:

Router(config)# interface Gig0/0
Router(config-if)# ip tcp adjust-mss 1360

Result: MTU mismatch can cause data loss especially during L2 (frame) and L3 (packet) transitions. It's not noticeable with small packets like ping, but network problems occur during large data transmissions. Therefore, MTU values need to be carefully configured in tunnel structures or service provider transitions.

Note: This problem demonstrates the interaction between Layer 2 (Data Link - frame size limits) and Layer 3 (Network - packet fragmentation) of the OSI model.