NAT

2.1 Explain characteristics of routing technologies

Address Translation

📘CompTIA Network+ (N10-009)

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Why NAT is important in IT networks

1. Conserves Public IP Addresses
   • Many devices inside a company need internet access. NAT allows them to share a single public IP instead of assigning a public IP to each device.
   • Example: A company with 100 computers can use NAT to access the internet using just 1 public IP address.
2. Adds Security
   • NAT hides the internal network structure from the outside world. External users only see the public IP, not individual private IPs.
   • This reduces the risk of direct attacks on internal devices.
3. Allows Private IP Usage
   • Internal networks often use private IP ranges (like 192.168.x.x or 10.x.x.x). NAT allows these private IPs to communicate with external networks.

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How NAT works

When a device inside a private network sends a request to the internet:

1. The device sends a packet with its private IP as the source.
2. The NAT device (usually a router) replaces the private IP with its public IP and keeps a record (translation table) of which internal IP requested what.
3. The packet goes to the destination server on the internet.
4. When the response comes back, the NAT device looks up its table and forwards the response to the correct internal device.

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Types of NAT

1. Static NAT (One-to-One)
   • Maps one private IP to one public IP permanently.
   • Use case: A server inside a company needs to always be accessible from the internet.
   • Example: A web server with private IP 192.168.1.10 is mapped to public IP 203.0.113.10. Anyone accessing 203.0.113.10 reaches that server.
2. Dynamic NAT (Many-to-Many)
   • Maps private IPs to a pool of public IPs dynamically.
   • Only works while a public IP is available in the pool.
   • Example: Internal devices may get any available public IP from the pool when they try to access the internet.
3. PAT (Port Address Translation) / NAT Overload
   • Maps many private IPs to one public IP using different port numbers.
   • This is the most common NAT used in home and enterprise networks.
   • Example:
     • Internal device A: 192.168.1.5 sends a request → NAT maps it to 203.0.113.1:10001
     • Internal device B: 192.168.1.6 sends a request → NAT maps it to 203.0.113.1:10002
     • Both devices share the same public IP but use different port numbers to distinguish traffic.

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NAT Translation Table (How NAT tracks connections)

• NAT keeps a table to track which internal IP and port is associated with which external IP and port.
• Example table for PAT:

Private IP	Private Port	Public IP	Public Port
192.168.1.5	5000	203.0.113.1	10001
192.168.1.6	5000	203.0.113.1	10002

This table ensures that when a response comes back, NAT knows exactly which internal device should receive it.

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Key Exam Points for NAT

1. NAT is used for IP address conservation, security, and private network communication.
2. Static NAT – one-to-one mapping, used for servers needing consistent external access.
3. Dynamic NAT – many-to-many mapping, uses a pool of public IPs.
4. PAT / NAT Overload – many-to-one mapping using port numbers, commonly used in enterprises and homes.
5. NAT modifies source or destination IP addresses but keeps track of connections to route responses correctly.
6. NAT is a layer 3 function (works at the IP layer) but also uses ports for PAT (layer 4).
7. NAT hides internal IPs from external networks, improving security.

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Example Scenario in IT Network

• A company has 50 employees using private IPs (10.1.1.x).
• The company only has 1 public IP.
• The NAT device converts each employee’s private IP into the public IP with a unique port number when accessing the internet.
• External servers only see the public IP 203.0.113.1, and the NAT table ensures that each response goes to the correct employee’s computer.

This explanation covers everything you need for the NAT section of the exam: concepts, types, tables, and key points.

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