Practical 5: Subnetting a Class B Network

You have been assigned the Class B network:

• Network Address: 172.16.0.0
• Default Subnet Mask: 255.255.0.0 (/16)
• Required Subnets: 8

Step 1 — Determine bits to borrow

How many host bits must you borrow to create at least 8 subnets?

• 2^x ≥ 8 → x = 3 (since 2³ = 8)

Step 2 — New subnet mask

• Original mask: /16 → borrow 3 more bits → /19
• Binary: 11111111.11111111.11100000.00000000
• Decimal: 255.255.224.0

Step 3 — Hosts per subnet

Work out the following from the /19 subnet mask:

Step 4 — Block size

• Block size in 3rd octet: 256 − 224 = 32
• Subnets increment by 32 in the 3rd octet: 0, 32, 64, 96, 128, 160, 192, 224

Step 5 — Complete the subnet table

Fill in the details for each of the 8 subnets.

Create the following network in Packet Tracer. We will use a Router-on-a-Stick design: one physical cable carries traffic for all 8 subnets using VLANs (Virtual LANs).

Equipment

• 1 × Router — 2911
• 1 × Switch — 2960-24TT
• 16 × PC — (2 per subnet)

Cabling

1. Place the Router at the top of the workspace.
2. Place the Switch below the router.
3. Connect the router GigabitEthernet0/0 to the switch GigabitEthernet0/1 using a Copper Straight-Through cable.
4. Arrange 16 PCs in 8 pairs below the switch (label them PC1–PC16).
5. Connect each PC (FastEthernet0) to the switch using Copper Straight-Through cables:
   • PC1 → Switch Fa0/1  |  PC2 → Switch Fa0/2
   • PC3 → Switch Fa0/3  |  PC4 → Switch Fa0/4
   • PC5 → Switch Fa0/5  |  PC6 → Switch Fa0/6
   • PC7 → Switch Fa0/7  |  PC8 → Switch Fa0/8
   • PC9 → Switch Fa0/9  |  PC10 → Switch Fa0/10
   • PC11 → Switch Fa0/11  |  PC12 → Switch Fa0/12
   • PC13 → Switch Fa0/13  |  PC14 → Switch Fa0/14
   • PC15 → Switch Fa0/15  |  PC16 → Switch Fa0/16

Click on the Switch, go to the CLI tab, and paste the following configuration. This creates 8 VLANs (one per subnet), assigns PC ports to the correct VLAN, and sets up a trunk link to the router.

Part A — Enter privileged mode & global configuration

enable
configure terminal
hostname SubnetSwitch

• enable — enters privileged EXEC mode (gives you full access to configuration commands).
• configure terminal — enters global configuration mode where you can change switch settings.
• hostname SubnetSwitch — gives the switch a meaningful name so you can identify it in the CLI prompt.

Part B — Create VLANs

We create 8 VLANs — one for each subnet. A VLAN (Virtual LAN) logically separates ports on the same physical switch into isolated networks. Devices in VLAN 10 cannot talk to devices in VLAN 20 without going through the router, just as if they were on completely separate switches.

vlan 10
 name Subnet_1
vlan 20
 name Subnet_2
vlan 30
 name Subnet_3
vlan 40
 name Subnet_4
vlan 50
 name Subnet_5
vlan 60
 name Subnet_6
vlan 70
 name Subnet_7
vlan 80
 name Subnet_8

• vlan 10 — creates (or enters) VLAN number 10.
• name Subnet_1 — assigns a human-readable name. This is optional but makes show vlan brief much easier to read.
• We use VLAN IDs 10, 20, 30 … 80 (multiples of 10) by convention — it keeps things tidy and leaves room for future VLANs in between.

Part C — Assign ports to VLANs (access mode)

Each pair of PCs is plugged into two switch ports. We put those ports in access mode and assign them to the correct VLAN. An access port belongs to exactly one VLAN and strips the VLAN tag before sending frames to the PC (the PC never knows it is on a VLAN).

interface range FastEthernet0/1-2
 switchport mode access
 switchport access vlan 10
!
interface range FastEthernet0/3-4
 switchport mode access
 switchport access vlan 20
!
interface range FastEthernet0/5-6
 switchport mode access
 switchport access vlan 30
!
interface range FastEthernet0/7-8
 switchport mode access
 switchport access vlan 40
!
interface range FastEthernet0/9-10
 switchport mode access
 switchport access vlan 50
!
interface range FastEthernet0/11-12
 switchport mode access
 switchport access vlan 60
!
interface range FastEthernet0/13-14
 switchport mode access
 switchport access vlan 70
!
interface range FastEthernet0/15-16
 switchport mode access
 switchport access vlan 80

• interface range FastEthernet0/1-2 — selects ports Fa0/1 and Fa0/2 at the same time (saves typing).
• switchport mode access — tells the port it will connect to an end device and carry traffic for only one VLAN.
• switchport access vlan 10 — assigns these ports to VLAN 10 (Subnet 1). Any frame arriving on Fa0/1 or Fa0/2 is treated as VLAN 10 traffic.

Part D — Configure the trunk link to the router

The uplink from the switch to the router must carry traffic for all 8 VLANs over a single cable. We set this port to trunk mode. A trunk port tags every outgoing frame with its VLAN ID (using the 802.1Q standard) so the router knows which subnet the frame belongs to.

interface GigabitEthernet0/1
 switchport mode trunk

• interface GigabitEthernet0/1 — selects the Gigabit port connected to the router.
• switchport mode trunk — enables trunking. The switch will now add a 4-byte 802.1Q tag to every frame sent to the router, identifying which VLAN it came from.

Part E — Save the configuration

end
write memory

• end — exits back to privileged EXEC mode.
• write memory — saves the running configuration to NVRAM so it persists after a reboot.

Click on the Router, go to the CLI tab, and enter the following configuration. This creates 8 sub-interfaces on a single physical port — one per VLAN/subnet — each acting as the default gateway for its subnet.

Part A — Enter privileged mode & bring up the physical interface

enable
configure terminal
hostname SubnetRouter
!
interface GigabitEthernet0/0
 no shutdown

• enable / configure terminal — same as on the switch: enters global configuration mode.
• hostname SubnetRouter — names the router for easy identification.
• interface GigabitEthernet0/0 — selects the physical interface connected to the switch trunk.
• no shutdown — turns the interface on. By default, router interfaces are administratively shut down (unlike switch ports which default to up). Without this command, none of the sub-interfaces will work.

Part B — Create sub-interfaces (one per VLAN)

A sub-interface is a logical division of a physical interface. The router uses sub-interfaces to handle traffic from different VLANs on the same physical cable — this is the “router-on-a-stick” design. Each sub-interface needs two things:

1. VLAN encapsulation — tells the router which VLAN tag to look for.
2. IP address — becomes the default gateway for all PCs in that subnet.

interface GigabitEthernet0/0.10
 encapsulation dot1Q 10
 ip address 172.16.0.1 255.255.224.0
!
interface GigabitEthernet0/0.20
 encapsulation dot1Q 20
 ip address 172.16.32.1 255.255.224.0
!
interface GigabitEthernet0/0.30
 encapsulation dot1Q 30
 ip address 172.16.64.1 255.255.224.0
!
interface GigabitEthernet0/0.40
 encapsulation dot1Q 40
 ip address 172.16.96.1 255.255.224.0
!
interface GigabitEthernet0/0.50
 encapsulation dot1Q 50
 ip address 172.16.128.1 255.255.224.0
!
interface GigabitEthernet0/0.60
 encapsulation dot1Q 60
 ip address 172.16.160.1 255.255.224.0
!
interface GigabitEthernet0/0.70
 encapsulation dot1Q 70
 ip address 172.16.192.1 255.255.224.0
!
interface GigabitEthernet0/0.80
 encapsulation dot1Q 80
 ip address 172.16.224.1 255.255.224.0

• interface GigabitEthernet0/0.10 — creates sub-interface .10 on the GigE0/0 port. The “.10” is just a label — we match it to the VLAN number by convention to keep things clear.
• encapsulation dot1Q 10 — tells this sub-interface to handle frames tagged with VLAN 10 (the 802.1Q standard). When a frame arrives from the switch with a VLAN 10 tag, the router sends it to this sub-interface for processing.
• ip address 172.16.0.1 255.255.224.0 — assigns the first usable IP in Subnet 1 as the gateway address. PCs in Subnet 1 will set 172.16.0.1 as their default gateway.

The same pattern repeats for all 8 sub-interfaces. Each one uses the next subnet’s first address (172.16.32.1, 172.16.64.1, … 172.16.224.1) and maps to the corresponding VLAN (20, 30, … 80).

Part C — Save the configuration

end
write memory

• end — exits back to privileged EXEC mode.
• write memory — saves the running configuration to NVRAM so it survives a reboot.

Assign a static IP address, subnet mask, and default gateway to each PC. The gateway is the router sub-interface IP for that subnet.

Configure each PC

1. Click on the PC → Desktop tab → IP Configuration.
2. Select Static.
3. Enter the IPv4 Address, Subnet Mask, and Default Gateway from the table above.
4. Close the window and repeat for all 16 PCs.

A — Ping within the same subnet

Open the Command Prompt on PC1 and ping PC2 (same subnet):

ping 172.16.0.3

This should succeed immediately — both PCs are in the same VLAN and do not need the router.

B — Ping across different subnets

From PC1 (Subnet 1), ping PC3 in Subnet 2:

ping 172.16.32.2

This traffic goes: PC1 → Switch → Router (routes between VLANs) → Switch → PC3.

C — Test more cross-subnet pings

1. PC5 → PC11: ping 172.16.160.2 (Subnet 3 → Subnet 6)
2. PC7 → PC15: ping 172.16.224.2 (Subnet 4 → Subnet 8)
3. PC16 → PC1: ping 172.16.0.2 (Subnet 8 → Subnet 1)

All pings should return successful replies.

D — Verify on the Router

On the router CLI, run:

show ip interface brief

You should see all 8 sub-interfaces with status up/up and their correct IP addresses.

1. Why did we borrow 3 bits and not 2 or 4?
2. What would change if you needed 16 subnets instead of 8?
3. Can PC1 (172.16.0.2) communicate with PC3 (172.16.32.2) without the router? Why or why not?
4. What is the purpose of the encapsulation dot1Q command on the router sub-interfaces?
5. How many usable host addresses does each /19 subnet provide? Is this efficient for a network with only 2 devices per subnet?
6. If you changed the subnet mask to /21, how many subnets would you have and how many hosts per subnet?

In this practical you have:

• Subnetted a Class B network (172.16.0.0/16) into 8 subnets using /19
• Calculated network, broadcast, and host ranges for all 8 subnets
• Built a router-on-a-stick topology in Packet Tracer
• Configured VLANs on a switch and sub-interfaces on a router
• Assigned static IPs with correct gateways to 16 PCs
• Verified intra-subnet and inter-subnet connectivity with ping