🔐 ACL (Access Control List)

ACL is a set of rules used on routers and switches to permit or deny network traffic based on IP addresses, protocols, or ports.



ACL is used for:

• Security
• Traffic filtering
• Blocking users
• Allowing specific networks
• Firewall-like control

Used in:

• Enterprises
• Firewalls
• ISPs
• Corporate networks
• Banks

🎯 Real-Life Example

Suppose company says:

❌ HR department cannot access Server
✅ IT department can access Server

👉 ACL handles this.

🔥 What You Will Learn

✔ Permit/Deny traffic
✔ Standard ACL
✔ Extended ACL basics
✔ Interface filtering
✔ Inbound/Outbound traffic
✔ Enterprise security logic

🧪 Practical Topology

In Cisco Packet Tracer create:

PC0 ---- Switch ---- Router ---- Server

🌐 IP Addressing

🔌 Wiring

Use:
✅ Copper Straight-Through

 ⚙️ Step 1: Configure Router Interfaces

enable
configure terminal

interface g0/0
ip address 192.168.1.1 255.255.255.0
no shutdown

interface g0/1
ip address 200.1.1.1 255.255.255.0
no shutdown

 ⚙️ Step 2: Configure PC & Server

🧪 Step 3: Test Before ACL

From PC0:
ping 200.1.1.2
✔ Success

🔥 Step 4: Create ACL

Now we block PC0 from reaching Server.

🔹 Create Standard ACL

access-list 1 deny 192.168.1.10
access-list 1 permit any

🧠 Meaning

🔥 Step 5: Apply ACL to Interface

Apply on router interface:

interface g0/0
ip access-group 1 in

🧠 Important Concept

in means:

Traffic entering interface.

🔍 Packet Flow

PC0 → Router G0/0 → ACL checks → deny
Packet dropped.

🧪 Step 6: Test Again

From PC0:
ping 200.1.1.2
❌ Failed
ACL blocked it.

  

🔥 Verify ACL

Run:
show access-lists
You’ll see packet matches.

🔥 Full ACL Configuration

enable
configure terminal

access-list 1 deny 192.168.1.10
access-list 1 permit any

interface g0/0
ip access-group 1 in

🧠 VERY IMPORTANT RULE

ACL processes:

TOP → DOWN

First match wins.

🔥 Hidden Rule

Every ACL ends with:

implicit deny any

Means:
If not permitted → automatically denied.

🎯 Standard vs Extended ACL

🔥 Example Extended ACL

access-list 101 deny tcp 192.168.1.0 0.0.0.255 any eq 80

Blocks:

HTTP traffic

 

🧠 Important Interview Questions

❓ What is ACL?

 Traffic filtering/security mechanism.

❓ What is implicit deny?
Hidden deny rule at end of ACL.

❓ Difference between inbound and outbound?

❓ Which ACL is closer to destination?
Standard ACL


❓ Which ACL is closer to source?
Extended ACL

Disclaimer

NAT (Network Address Translation)

NAT is a process that translates private IP addresses into public IP addresses to enable communication with external networks like the internet. NAT Network Address Translation practical in cisco packet tracer

This is used in:

• Home WiFi routers
• Offices
• Enterprises
• ISPs
• Firewalls

Without NAT:

👉 Private IP devices cannot access the internet.

🎯 Why NAT Exists

Private IPs:

• 192.168.x.x
• 10.x.x.x
• 172.16.x.x

❌ Cannot work directly on internet.

So router does:

Private IP  →  Public IP

🔥 What You Will Learn

• ✔ Inside vs Outside interfaces
• ✔ Private/Public IP
• ✔ NAT translation
• ✔ Internet simulation
• ✔ PAT overload
• ✔ Enterprise internet access

🧪 Practical Topology

In Cisco Packet Tracer create:

PC0 ---- Switch ---- Router ---- Server

🌐 IP Addressing

🔹 Internal LAN

🔹 External Network

🔌 Wiring

PC → Switch

Use:

✅ Copper Straight-Through

Switch → Router

Use:

✅ Copper Straight-Through

Router → Server

Use:

✅ Copper Straight-Through

🔥 Physical Port Connections

Like - 

⚙️ Step 1: Configure Router Interfaces

enable
configure terminal

interface g0/0
ip address 192.168.1.1 255.255.255.0
ip nat inside
no shutdown

interface g0/1
ip address 200.1.1.1 255.255.255.0
ip nat outside
no shutdown

🧠 Important Concept

⚙️ Step 2: Configure PC

⚙️ Step 3: Configure Server

🔥 Step 4: Create Access List

access-list 1 permit 192.168.1.0 0.0.0.255

🧠 Why ACL?

Tells router:

👉 Which internal IPs should be translated.

🔥 Step 5: Enable NAT Overload (PAT)

ip nat inside source list 1 interface g0/1 overload

🧠 Meaning

🎯 Full NAT Configuration

enable
configure terminal

interface g0/0
ip address 192.168.1.1 255.255.255.0
ip nat inside
no shutdown

interface g0/1
ip address 200.1.1.1 255.255.255.0
ip nat outside
no shutdown

access-list 1 permit 192.168.1.0 0.0.0.255

ip nat inside source list 1 interface g0/1 overload

🧪 Step 6: Test Connectivity

From PC0:

ping 200.1.1.2

✔ Success

🔍 What Happens Internally

192.168.1.10
↓
Router translates
↓
200.1.1.1
↓
Server

🔥 Verify NAT Translation

On router:

show ip nat translations

You’ll see:

Inside local → Inside global

🔥 Most Important Interview Questions

❓ Difference Between NAT and PAT?

❓ What is Overload?

PAT (Port Address Translation)

❓ Why NAT Important?

Conserves public IPv4 addresses.

❓ What command verifies NAT?

show ip nat translations

🔥 Common Mistakes

🎯 What You Learned

• ✔ NAT basics
• ✔ Private/Public IP
• ✔ PAT overload
• ✔ ACL for NAT
• ✔ Enterprise internet logic

Disclaimer

🌐 OSPF (Open Shortest Path First)

OSPF is a link-state dynamic routing protocol that automatically exchanges routing information between routers and calculates the shortest path using the SPF algorithm.

This is dynamic routing.

Until now in static routing:

👉 You manually added routes.

Now with OSPF:

👉 Routers automatically learn routes from each other.

This is heavily used in:

• Enterprise networks
• ISPs
• Data centers
• Corporate environments

🎯 What You Will Learn

• ✔ Dynamic route learning
• ✔ Neighbor relationships
• ✔ Route advertisement
• ✔ Automatic path selection
• ✔ Scalable routing

🔥 Difference: Static vs OSPF

🧪 Practical Topology

In Cisco Packet Tracer create:

PC0 --- R1 --- R2 --- R3 --- PC1

🌐 IP Addressing Plan

🔹 LAN Networks

🔹 Router-to-Router Links

🔌 Wiring (IMPORTANT)

PC Connections

Router Connections

⚙️ Step 1: Configure Router Interfaces

🔹 Router1

enable
configure terminal

interface g0/0
ip address 10.0.0.1 255.255.255.0
no shutdown

interface g0/1
ip address 192.168.1.1 255.255.255.252
no shutdown

🔹 Router2

enable
configure terminal

interface g0/0
ip address 192.168.1.2 255.255.255.252
no shutdown

interface g0/1
ip address 192.168.2.1 255.255.255.252
no shutdown

🔹 Router3

enable
configure terminal

interface g0/0
ip address 192.168.2.2 255.255.255.252
no shutdown

interface g0/1
ip address 20.0.0.1 255.255.255.0
no shutdown

⚙️ Step 2: Configure PCs

PC0

IP: 10.0.0.10

Gateway: 10.0.0.1

PC1

IP: 20.0.0.10

Gateway: 20.0.0.1

❌ Test Before OSPF

From PC0:

ping 20.0.0.10

❌ Fail

Because routers don’t know remote networks yet.

🔥 Step 3: Configure OSPF

🔹 Router1

router ospf 1

network 10.0.0.0 0.0.0.255 area 0
network 192.168.1.0 0.0.0.3 area 0

🔹 Router2

router ospf 1

network 192.168.1.0 0.0.0.3 area 0
network 192.168.2.0 0.0.0.3 area 0

🔹 Router3

router ospf 1

network 192.168.2.0 0.0.0.3 area 0
network 20.0.0.0 0.0.0.255 area 0

🧠 Important Concept

What is Area 0?

👉 Backbone area of OSPF

All routers communicate inside Area 0.

🔥 Step 4: Verify Neighbor Relationship

Run:

show ip ospf neighbor

You should see neighboring routers.

🔥 Step 5: Verify Routing Table

Run:

show ip route

You’ll see routes marked:

O

👉 Means learned through OSPF

🧪 Final Test

From PC0:

ping 20.0.0.10

✔ SUCCESS

Disclaimer

🌐 DHCP (Dynamic Host Configuration Protocol)

DHCP is a network protocol used to automatically assign IP configuration to clients in a network.

This is used in:

• Offices
• Companies
• Schools
• WiFi routers
• ISPs
• Enterprise networks

Almost every modern network uses DHCP.

🎯 What DHCP Does

Instead of manually assigning IP addresses to every PC:

❌ Manual:

PC1 = 192.168.1.10
PC2 = 192.168.1.11
PC3 = 192.168.1.12

✅ DHCP automatically gives:

• IP Address
• Subnet Mask
• Default Gateway
• DNS Server

🔥 Real-Life Example

When you connect:

Mobile to WiFi

Laptop to office LAN

👉 Router automatically gives IP

That is DHCP.

🎯 What You Will Learn

• ✔ Automatic IP allocation
• ✔ DHCP pool creation
• ✔ Default gateway assignment
• ✔ DNS assignment
• ✔ Lease process
• ✔ Enterprise networking basics

🧪 Practical Topology

In Cisco Packet Tracer create:

PC0 ----\
PC1 ----- Switch ---- Router
PC2 ----/

🌐 IP Plan

🔌 Wiring

PCs → Switch

Use:

Copper Straight-Through

Switch → Router

Use:

Copper Straight-Through

⚙️ Step 1: Configure Router Interface

On router:

enable
configure terminal

interface g0/0
ip address 192.168.1.1 255.255.255.0
no shutdown

🔥 Step 2: Configure DHCP Service

ip dhcp pool OFFICE

👉 Creates DHCP pool named OFFICE

Configure Network

network 192.168.1.0 255.255.255.0

Configure Default Gateway

default-router 192.168.1.1

Configure DNS

dns-server 8.8.8.8

🔥 Full DHCP Configuration

enable
configure terminal

interface g0/0
ip address 192.168.1.1 255.255.255.0
no shutdown

ip dhcp pool OFFICE
network 192.168.1.0 255.255.255.0
default-router 192.168.1.1
dns-server 8.8.8.8

🔥 Step 3: Exclude Router IP

VERY IMPORTANT

ip dhcp excluded-address 192.168.1.1

🧠 Why?

👉 Prevents DHCP from assigning router IP to PCs.

🔥 Step 4: Configure PCs

On every PC:

Desktop → IP Configuration

Select:

DHCP

🎯 Result

PC automatically gets:

🧪 Step 5: Verify

From PC:

ipconfig

You’ll see DHCP-assigned IP.

🧪 Step 6: Test Connectivity

ping 192.168.1.1

✔ Success

🔍 DHCP Process (VERY IMPORTANT)

DORA Process

🧠 How It Works

• 1️⃣ PC broadcasts DHCP Discover
• 2️⃣ Router replies DHCP Offer
• 3️⃣ PC sends Request
• 4️⃣ Router sends Acknowledgement

✔ IP assigned

🔥 Verification Commands

On router:

show ip dhcp binding

👉 Shows assigned IPs

show ip dhcp pool

👉 Shows DHCP pool details

🧠 Important Interview Questions

❓ What port does DHCP use?

UDP 67 & 68

❓ What is APIPA?

If DHCP fails:

169.254.x.x

❓ Why exclude addresses?

Prevent important IP assignment conflicts.

❓ What is lease time?

Duration client can use IP.

🔥 Common Mistakes

🎯 What You Learned

• ✔ Automatic IP assignment
• ✔ DHCP pool
• ✔ DORA process
• ✔ Enterprise IP management
• ✔ Router as DHCP server

Disclaimer