Practical 6: Subnetting a Class A Network & IPv6 Dual-Stack

You have been assigned the Class A network:

• Network Address: 10.0.0.0
• Default Subnet Mask: 255.0.0.0 (/8)
• Required Subnets: 1024

Step 1 — Determine bits to borrow

How many host bits must you borrow to create at least 1024 subnets?

• 2^x ≥ 1024 → x = 10 (since 2¹⁰ = 1024)

Step 2 — New subnet mask

• Original mask: /8 → borrow 10 more bits → /18
• Binary: 11111111.11111111.11000000.00000000
• Decimal: 255.255.192.0

Step 3 — Hosts per subnet

Work out the following from the /18 subnet mask:

Question	Your Answer
Remaining host bits
Total addresses per subnet (2?)
Usable hosts per subnet (2? − 2)

✔ Check

Step 4 — Block size

• Block size in 3rd octet: 256 − 192 = 64
• Subnets increment by 64 in the 3rd octet: 0, 64, 128, 192 (then the 2nd octet increments)
• Pattern: 10.0.0.0, 10.0.64.0, 10.0.128.0, 10.0.192.0, 10.1.0.0, 10.1.64.0, …

Step 5 — Complete the subnet table

Fill in the details for the first 4 subnets and the last subnet (1024th). With 1024 subnets we cannot list them all — but the pattern is the same throughout.

✔ Check Answers

We will build a topology using the first 3 subnets only (subnets 1, 2, and 3). This keeps the lab manageable while demonstrating the same concepts. We use a Router-on-a-Stick design with VLANs, just like Practical 5.

Equipment

• 1 × Router — 2911
• 1 × Switch — 2960-24TT
• 6 × 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 6 PCs in 3 pairs below the switch (label them PC1–PC6).
5. Connect each PC (FastEthernet0) to the switch using Copper Straight-Through cables:
   • PC1 → Switch Fa0/1  |  PC2 → Switch Fa0/2   (Subnet 1)
   • PC3 → Switch Fa0/3  |  PC4 → Switch Fa0/4   (Subnet 2)
   • PC5 → Switch Fa0/5  |  PC6 → Switch Fa0/6   (Subnet 3)

Click on the Switch, go to the CLI tab, and enter the following configuration. This creates 3 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 ISP-Switch

Part B — Create VLANs

vlan 10
 name Subnet_1
vlan 20
 name Subnet_2
vlan 30
 name Subnet_3

• vlan 10 — creates VLAN 10 for Subnet 1 (10.0.0.0/18).
• name Subnet_1 — gives the VLAN a readable label.

Part C — Assign ports to VLANs

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 FastEthernet0/1-2 — selects both ports for Subnet 1 PCs.
• switchport mode access — sets the port as an access port (single VLAN).
• switchport access vlan 10 — assigns these ports to VLAN 10.

Part D — Configure the trunk link

interface GigabitEthernet0/1
 switchport mode trunk

Part E — Save the configuration

end
write memory

Click on the Router, go to the CLI tab, and create 3 sub-interfaces — one per VLAN — with IPv4 addresses. Each sub-interface acts as the default gateway for its subnet.

Part A — Hostname & bring up physical interface

enable
configure terminal
hostname ISP-Router
!
interface GigabitEthernet0/0
 no shutdown

Part B — Create sub-interfaces (IPv4)

interface GigabitEthernet0/0.10
 encapsulation dot1Q 10
 ip address 10.0.0.1 255.255.192.0
!
interface GigabitEthernet0/0.20
 encapsulation dot1Q 20
 ip address 10.0.64.1 255.255.192.0
!
interface GigabitEthernet0/0.30
 encapsulation dot1Q 30
 ip address 10.0.128.1 255.255.192.0

• encapsulation dot1Q 10 — maps this sub-interface to VLAN 10 (802.1Q tagging).
• ip address 10.0.0.1 255.255.192.0 — assigns the first usable host address in Subnet 1 as the gateway. PCs in Subnet 1 will use 10.0.0.1 as their default gateway.
• Subnet 2 gateway: 10.0.64.1  |  Subnet 3 gateway: 10.0.128.1

Part C — Save the configuration

end
write memory

Now we add IPv6 addresses to the same router sub-interfaces that already have IPv4 addresses. This is called dual-stack — both protocols run simultaneously on the same interfaces. Devices can communicate using either IPv4 or IPv6.

IPv6 addressing plan

VLAN	IPv6 Network	Router Gateway (::1)
10 (Subnet 1)	2001:DB8:ACAD:1::/64	2001:DB8:ACAD:1::1
20 (Subnet 2)	2001:DB8:ACAD:2::/64	2001:DB8:ACAD:2::1
30 (Subnet 3)	2001:DB8:ACAD:3::/64	2001:DB8:ACAD:3::1

Part A — Enable IPv6 routing

By default, Cisco routers do not route IPv6 traffic. You must enable it first:

enable
configure terminal
ipv6 unicast-routing

• ipv6 unicast-routing — enables the router to forward IPv6 packets between interfaces. Without this command, the router will accept IPv6 addresses on interfaces but will not route traffic between them.

Part B — Add IPv6 addresses to sub-interfaces

interface GigabitEthernet0/0.10
 ipv6 address 2001:DB8:ACAD:1::1/64
!
interface GigabitEthernet0/0.20
 ipv6 address 2001:DB8:ACAD:2::1/64
!
interface GigabitEthernet0/0.30
 ipv6 address 2001:DB8:ACAD:3::1/64

• ipv6 address 2001:DB8:ACAD:1::1/64 — assigns an IPv6 address to the sub-interface. Notice this is in addition to the IPv4 address that was already configured — both coexist.
• The ::1 at the end means the interface ID is 0000:0000:0000:0001 — we use ::1 as the gateway by convention, similar to .1 in IPv4.

Part C — Save the configuration

end
write memory

Assign both IPv4 and IPv6 static addresses to each PC. Go to each PC → Desktop → IP Configuration.

IPv4 addressing

IPv6 addressing

On the same IP Configuration screen, scroll down to the IPv6 Configuration section. Select Static and enter:

A — IPv4: Ping within the same subnet

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

ping 10.0.0.3

This should succeed immediately — both PCs are in the same VLAN.

B — IPv4: Ping across different subnets

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

ping 10.0.64.2

This traffic goes: PC1 → Switch (VLAN 10) → Router → Switch (VLAN 20) → PC3.

C — IPv4: More cross-subnet pings

1. PC1 → PC5: ping 10.0.128.2 (Subnet 1 → Subnet 3)
2. PC3 → PC6: ping 10.0.128.3 (Subnet 2 → Subnet 3)
3. PC6 → PC1: ping 10.0.0.2 (Subnet 3 → Subnet 1)

D — IPv6: Ping within the same subnet

From PC1, ping PC2 using IPv6:

ping 2001:DB8:ACAD:1::11

E — IPv6: Ping across subnets

From PC1 (Subnet 1), ping PC3 in Subnet 2 using IPv6:

ping 2001:DB8:ACAD:2::10

More IPv6 cross-subnet tests:

1. PC1 → PC5: ping 2001:DB8:ACAD:3::10 (Subnet 1 → Subnet 3)
2. PC4 → PC6: ping 2001:DB8:ACAD:3::11 (Subnet 2 → Subnet 3)

F — Verify on the Router

On the router CLI, run both commands:

show ip interface brief
show ipv6 interface brief

You should see all 3 sub-interfaces with status up/up and their correct IPv4 and IPv6 addresses.

1. Why did we borrow 10 bits and not 8 or 12?
2. How many usable hosts does each /18 subnet support? Is this more or fewer than the /19 subnets from Practical 5?
3. If you only needed 512 subnets, how many bits would you borrow and what would the new mask be?
4. What is the purpose of ipv6 unicast-routing? What happens if you forget it?
5. Can a PC with only an IPv4 address ping a PC that only has an IPv6 address? Why or why not?
6. What are two advantages of running dual-stack on a network?

In this practical you have:

• Subnetted a Class A network (10.0.0.0/8) into 1024 subnets using /18
• Calculated network, broadcast, and host ranges for selected subnets
• Built a router-on-a-stick topology in Packet Tracer with 3 VLANs
• Configured VLANs on a switch and IPv4 sub-interfaces on a router
• Enabled IPv6 unicast routing and assigned IPv6 addresses (dual-stack)
• Assigned static IPv4 and IPv6 addresses to PCs
• Verified connectivity using ping for both IPv4 and IPv6