5.5 NAT

Network Address Translation (NAT) is a technique used in networking to translate private IP addresses used in a local network into a public IP address before packets are forwarded to the internet. This allows multiple devices on a local network to share a single public IP address.

✅ Why Use NAT?

• Conserves public IP addresses.
• Adds a layer of security by hiding internal IPs.
• Required when private IPs (like 192.168.x.x) need to access the internet.

🔧 Types of NAT

• Static NAT – One-to-one mapping of internal to public IP.
• Dynamic NAT – Pool of public IPs assigned as needed.
• PAT (Port Address Translation) – Many-to-one using ports (most common).

1. Configuring NAT

Play

NAT - Part 01

Play

NAT - Part 02

Configuring NAT This document describes how to configure the Network Address Translation (NAT) on a Cisco router.

2. 📘 Simple NAT Example in Cisco Packet Tracer

🎯 Goal: PC in a private network (192.168.1.x) accesses the internet using a router with NAT.

🖥 Network Setup

Device	IP Address	Interface
PC0	192.168.1.10	---
Router	Fa0/0: 192.168.1.1 (Inside)	Fa0/1: 203.0.113.2 (Outside)
Server	203.0.113.5 (Public Web Server)	---

🛠 Step-by-Step in Packet Tracer

1. Add Devices

• 1 PC
• 1 Router
• 1 Server
• 2 Switches (optional, for realism)

2. Configure IPs

PC0:

IP: 192.168.1.10
Subnet: 255.255.255.0
Gateway: 192.168.1.1

Router:

interface fa0/0
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
 no shutdown

interface fa0/1
 ip address 203.0.113.2 255.255.255.0
 ip nat outside
 no shutdown

Server:

IP: 203.0.113.5
Subnet: 255.255.255.0

3. Configure NAT on Router

access-list 1 permit 192.168.1.0 0.0.0.255

ip nat inside source list 1 interface fa0/1 overload

🔁 This means:

• NAT will translate packets from 192.168.1.x range
• It will use the public IP of interface fa0/1
• Overload means multiple internal IPs can share one public IP using ports (PAT)

4. Test

• From PC0, open Command Prompt
• Type: ping 203.0.113.5

If NAT is configured correctly, the ping will succeed.

In networking, ACLs (Access Control Lists) are rules used to control the flow of traffic into or out of a network. They’re used mainly on routers, switches, and firewalls to filter traffic based on specified conditions.

🔐 What an ACL Does

An ACL defines what kind of traffic is allowed or denied. Each rule in the list checks packet attributes like:

• Source IP address
• Destination IP address
• Port numbers (e.g., TCP/UDP ports)
• Protocol (e.g., TCP, UDP, ICMP)

📋 Types of ACLs

1. Standard ACL

   • Filters only by source IP address
   • Example: Allow traffic from 192.168.1.0/24

2. Extended ACL

   • Filters by source/destination IP, ports, and protocol
   • More precise control
   • Example: Allow HTTP traffic from 192.168.1.0/24 to 10.0.0.0/24

3. Named ACL

   • Same as standard/extended but uses a name instead of a number

⚙️ Where ACLs Are Applied

• Inbound: Before the router processes the packet
• Outbound: After processing, before forwarding

✅ Example (Cisco-style)

access-list 100 permit tcp 192.168.1.0 0.0.0.255 any eq 80

This allows HTTP (TCP port 80) traffic from 192.168.1.0/24 to any destination.

IMPORTANT

ACLs are like traffic bouncers. They check the packet “ID” and decide whether to let it in or kick it out.

1. Standard ACL

Play

ACL - 01

Play

ACL - 02

Play

ACL - 02

Types of ACL Various types of IP Access Control Lists (ACLs) and how they can filter network traffic.

Wildcard Bits Explained When you are studying Cisco and access-lists you will encounter the so-called Wildcard Bits.

2. Extended ACL

Play

Extended ACL - 01

Play

Extended ACL - 02

Configuring Extended ACLs (Access Lists) To be more precise when matching a certain network traffic, extended access lists are used.