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Lesson 1 of 6

Subnet Mask Fundamentals

Lab Objectives

  • Understand subnet masks, CIDR notation, and how to convert between binary and decimal.
  • Calculate network, broadcast, and host ranges for common masks.
  • Practice applying masks by configuring loopback interfaces to represent subnet gateways and verifying with show commands.

Use the BASE LAB TOPOLOGY exactly as shown:

                [Internet]
               203.0.113.1
                    |
               R1 (Gateway)
              Gi0/0: 10.10.10.1
              Gi0/1: 10.10.20.1
              Gi0/2: 10.10.30.1
              /     |     \
           R2      R3      R4

Gi0/0: 10.10.10.2 | Gi0/0: 10.10.30.2 Gi0/1: 10.10.40.1 | / \ | S1 S2 S3 / \ | /
PC1 PC2 PC3 PC4 PC5

IP SCHEME (use these subnets during the lab):

  • 10.10.10.0/24 — R1-R2 link
  • 10.10.20.0/24 — R1-R3 link
  • 10.10.30.0/24 — R1-R4 link
  • 10.10.40.0/24 — R2-S1 link
  • 192.168.1.0/24 — VLAN 10 (Sales)
  • 192.168.2.0/24 — VLAN 20 (Engineering)
  • 192.168.3.0/24 — VLAN 30 (Management)
  • 203.0.113.0/24 — Public/Internet simulation

Tip: Think of a subnet mask like a cookie cutter on the IP address: it defines which portion of the IP is the network (the cutter shape) and which portion is available for hosts. CIDR (/ notation) is simply a compact way to describe how many bits are used by the cookie cutter.

Lab Tasks (Try It Yourself First!)

Complete these tasks WITHOUT looking at the solution below. Use ? and show commands to figure it out.

Task 1: Configure a /24 gateway for VLAN 10 (Sales)

Configure a loopback interface on R2 to represent the VLAN 10 gateway: use IP 192.168.1.1 and a /24 mask (255.255.255.0). This models a router SVI or gateway for that VLAN.

Task 2: Configure a /24 gateway for VLAN 20 (Engineering)

Configure a loopback interface on R3 with IP 192.168.2.1/24 to represent the VLAN 20 gateway.

Task 3: Practice CIDR and binary conversion for VLAN 30 (Management)

Divide 192.168.3.0/24 into four /26 subnets. Configure a loopback on R4 to use the second /26 subnet's gateway address (192.168.3.65 with mask 255.255.255.192). Explain how you derived the network, broadcast, and host range for that /26.

Think About It: Why can two /26 subnets created from a /24 coexist without overlapping addresses, and what happens if you accidentally configure a gateway with the wrong mask?

Lab Solution

Task 1 Solution: Configure a /24 gateway for VLAN 10 (Sales)

What we are doing: Creating a loopback interface on R2 to represent the gateway for VLAN 10 using 192.168.1.1 with mask 255.255.255.0 (/24). This isolates the concept of network, broadcast, and hosts without touching switch SVIs.

R2(config)# interface loopback0
R2(config-if)# ip address 192.168.1.1 255.255.255.0
R2(config-if)# exit
  • What the commands do:
    • interface loopback0 — creates a logical loopback interface on the router (useful as a stable gateway in labs).
    • ip address 192.168.1.1 255.255.255.0 — assigns the IP and mask to the loopback.
  • Why they matter:
    • Using a loopback isolates the subnet configuration from physical link changes and demonstrates how a router advertises and hosts use a gateway.
    • /24 mask means 24 bits network, 8 bits for hosts — standard class C sized network.

Verify:

R2# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     10.10.10.2      YES manual up                    up
GigabitEthernet0/1     10.10.40.1      YES manual up                    up
Loopback0              192.168.1.1     YES manual up                    up


<div class="topology-diagram">
<img src="data:image/svg+xml;base64,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" alt="Network Topology Diagram" style="max-width:100%;height:auto;background:#fff;padding:16px;border:1px solid #e5e7eb;border-radius:8px;" />
</div>

cisco
R3(config)# interface loopback0
R3(config-if)# ip address 192.168.2.1 255.255.255.0
R3(config-if)# exit
  • What the commands do:
    • Adds loopback0 and assigns the /24 address.
  • Why they matter:
    • Consistent /24 networks make host addressing simple; each VLAN typically maps to a single IP subnet.

Verify:

R3# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     10.10.20.2      YES manual up                    up
Loopback0              192.168.2.1     YES manual up                    up


<div class="topology-diagram">
<img src="data:image/svg+xml;base64,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" alt="Network Topology Diagram" style="max-width:100%;height:auto;background:#fff;padding:16px;border:1px solid #e5e7eb;border-radius:8px;" />
</div>

cisco
R4(config)# interface loopback0
R4(config-if)# ip address 192.168.3.65 255.255.255.192
R4(config-if)# exit
  • What the commands do:
    • Create a loopback and assign the address inside the second /26 subnet.
  • Why they matter:
    • Demonstrates converting mask to CIDR and binary ( /26 = 26 ones ), calculating subnet increments (64), and choosing a gateway inside the host range.

Verify:

R4# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     10.10.30.2      YES manual up                    up
Loopback0              192.168.3.65    YES manual up                    up


<div class="topology-diagram">
<img src="data:image/svg+xml;base64,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" alt="Network Topology Diagram" style="max-width:100%;height:auto;background:#fff;padding:16px;border:1px solid #e5e7eb;border-radius:8px;" />
</div>

cisco
R4# show running-config interface loopback0
  • If you see ip address 192.168.3.65 255.255.255.0, correct it:
R4(config)# interface loopback0
R4(config-if)# ip address 192.168.3.65 255.255.255.192
  • Why: The wrong mask changes network boundaries and prevents correct ARP and routing behavior. With the proper /26, the router and hosts agree on the network and host ranges.

Verification Checklist

  • Loopback0 on R2 configured as 192.168.1.1 255.255.255.0
  • Loopback0 on R3 configured as 192.168.2.1 255.255.255.0
  • Loopback0 on R4 configured as 192.168.3.65 255.255.255.192
  • Understand how /26 divides a /24 into four subnets (increments of 64)

Common Mistakes

SymptomCauseFix
Loopback IP shows up but hosts cannot ping gatewayRouter mask differs from host mask (e.g., /24 vs /26)Correct the mask on router or host to match the intended subnet
Overlapping subnets after dividing a /24Incorrect increment calculation (used /28 math instead of /26)Recalculate using mask binary; /26 increments by 64
Chosen gateway is network or broadcast addressUsing .0 or .63 for /26 network incorrectlySelect an address within the host range (.1–.62 for 192.168.3.0/26; .65–.126 for 192.168.3.64/26)

Challenge Task

Divide 10.10.40.0/24 (R2–S1 link) into eight equal subnets (/27). Assign one /27 to host group A and configure a loopback on R2 with the first usable IP of that /27. Document network, broadcast, and usable host range for that /27. No step-by-step commands provided — design and configure independently.

Important: Understanding binary conversion and mask arithmetic is essential for CCNA-level subnetting questions on the exam. Practice converting between dotted-decimal masks and CIDR (e.g., 255.255.255.192 = /26) until it becomes second nature.

Practice with Hands-On Labs

30 step-by-step labs for CCNA, CCNP Enterprise & Security