Wrong subnet mask

5.3 Given a scenario, troubleshoot common issues with network services

Addressing Issues

📘CompTIA Network+ (N10-009)

---

A subnet mask is a number that works with an IP address to define which part of the address identifies the network and which part identifies the host (device) within that network. It’s essential for devices to communicate correctly on the same network and reach other networks through a router.

When a device has the wrong subnet mask, it can cause serious communication problems.

---

1. How a Subnet Mask Works

• Example IP: 192.168.1.10
• Example Subnet Mask: 255.255.255.0

Here:

• Network part: 192.168.1
• Host part: 10

The subnet mask tells the device:

• “Everything in 192.168.1.x is on the local network.”
• “Anything outside 192.168.1.x should go through the default gateway (router).”

---

2. What Happens if the Subnet Mask Is Wrong

If the subnet mask is incorrect, devices may:

1. Fail to communicate with local devices:
   • Example: Device A (192.168.1.10/255.255.255.0) tries to reach Device B (192.168.1.20/255.255.255.128).
   • Device A thinks Device B is on another network due to the wrong mask.
   • It sends traffic to the default gateway instead of directly communicating. Communication can fail if the gateway cannot route it.
2. Fail to reach devices on other networks:
   • If the subnet mask is too small (e.g., 255.255.255.192), the device might think some remote IPs are local.
   • It will try to reach them directly instead of using the router, and the traffic gets lost.
3. IP conflicts and connectivity issues:
   • Wrong subnet masks can make devices think other IP addresses are part of their network, which can create duplicate IP conflicts or unreachable devices.

---

3. Common Wrong Subnet Mask Scenarios

1. Too large a subnet (mask allows too many hosts):
   • Device thinks more devices are local than actually are.
   • Example: Using 255.255.0.0 on a network designed for 255.255.255.0.
   • Result: Devices send local traffic to machines far away instead of using a router → delays, packet loss.
2. Too small a subnet (mask allows too few hosts):
   • Device thinks fewer devices are local.
   • Example: Using 255.255.255.192 in a network that needs 255.255.255.0.
   • Result: Device sends local traffic to the gateway unnecessarily → connectivity problems.

---

4. How to Identify a Wrong Subnet Mask

• Ping test failure: You cannot reach local devices even though they are powered on and connected.
• IP configuration check:
  • On Windows: ipconfig /all
  • On Linux/macOS: ifconfig or ip addr
• Network segmentation mismatch:
  • Compare the subnet mask of all devices in the same network. They must match or be compatible.
• Error messages:
  • Some devices or applications may show “Destination unreachable” or “Network unreachable.”

---

5. How to Fix a Wrong Subnet Mask

1. Verify the network design:
   • Know which subnet mask is assigned to that network.
   • Check the IP address range and number of hosts needed.
2. Update the device configuration:
   • Static IP: Manually set the correct subnet mask.
   • DHCP: Ensure the DHCP server provides the correct subnet mask.
3. Check consistency:
   • All devices on the same subnet should have compatible subnet masks.
   • Ensure routers and gateways are configured for proper routing.

---

6. Key Exam Points

• Wrong subnet masks cause local communication failure and routing issues.
• Symptoms include:
  • Cannot ping local devices.
  • Cannot access devices on other subnets.
  • Unexpected network errors or unreachable hosts.
• Fixing involves verifying and correcting the subnet mask on devices and ensuring network consistency.
• Understand subnet masks in binary (for exam questions):
  • 255.255.255.0 = /24
  • 255.255.255.128 = /25
  • /24 allows 254 hosts, /25 allows 126 hosts, etc.

---

✅ Simple IT-focused example:

• You have a printer at 192.168.1.50/255.255.255.0 and a PC at 192.168.1.20/255.255.255.128.
• The PC thinks the printer is on a different network because of the wrong mask.
• It sends traffic to the router instead of directly to the printer → printing fails.
• Correcting the PC to 255.255.255.0 fixes the communication.

---

This covers everything you need to know about wrong subnet masks for the Network+ exam, explained in simple IT terms.