This document discusses Network Address Translation (NAT). It begins with an overview of NAT, describing it as a method of remapping an IP address space from one range to another by modifying packet headers. It then covers the different NAT types (static NAT, dynamic NAT, and PAT), how they work, and how to configure each type on a router. It also discusses verifying NAT configurations and the limited need for NAT with IPv6.

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8- NAT
8.1 NAT Overview
8.2 NAT Configuration
8.3 Troubleshooting NAT

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8.1 NAT Overview

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WHATISNAT?
• Network Address Translation
• Method of remapping one IP
address space into another by
modifying network address
information in Internet Protocol
(IP) datagram packet headers
while they are in transit across a
traffic routing device

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WHYWENEEDNAT?
• IPv4 address space is limited so
each device can’t use a global
address
• Private IPv4 address range is not
routed by internet routers so they
need to be translated to global
addresses
• Mostly enabled on core routers

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IPv4PRIVATEADDRESSRANGE
R1IPv4 Private Address Range IPv4 Public Address Range

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NATTERMINOLOGY
• Inside Local - Your host IP
• Inside Global - Your public IP
• Outside Local - Remote host IP
• Outside Global - Remote public IP
• Inside Local : 10.41.1.5
• Inside Global : 17.25.6.2
• Outside Local : 192.168.3.5
• Outside Global : 74.40.1.5
R1
SW1
Gig 0/0
PC1
R2
SW2
Gig 0/0
10.41.1.1
10.41.1.5
17.25.6.2 74.40.1.5
192.168.3.5
192.168.3.1

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NATTYPES
• Static NAT
• Dynamic NAT
• PAT

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STATICNAT
• Private addresses are
matched with global
addresses one to one R1
PC3
PC2
PC1
10.41.1.1
10.41.1.2
10.41.1.3
203.38.201.35
203.38.201.36
203.38.201.37
INSIDE LOCAL INSIDE GLOBAL
10.41.1.1 203.38.201.35
10.41.1.2 203.38.201.36
10.41.1.3 203.38.201.37
R1 NAT TABLE

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DYNAMICNAT
• Public IPv4 pool is
configured on router
• IP addresses are
assigned with first come
first served rule
R1
PC3
PC2
PC1
10.41.1.1
10.41.1.2
10.41.1.3
ADDRESS POOL
203.38.201.35
203.38.201.36
203.38.201.37
203.38.201.38
203.38.201.39
.
.

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PAT
• Port Address Translation
• Each private address is
translated to a single or a
few public IPv4 address
• Port addresses are used
for choosing the right
internal device when the
packet is coming back
R1
PC3
PC2
PC1
10.41.1.1
10.41.1.2
10.41.1.3
203.38.201.35

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8.2 NAT Configuration

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STATICNATCONFIGURATION
R1
PC3
PC2
PC1
10.41.1.1
10.41.1.2
10.41.1.3
203.38.201.35
203.38.201.36
203.38.201.37
Gig 0/0 Gig 0/1

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VERIFYINGSTATICNATCONFIGURATION
R1
PC3
PC2
PC1
10.41.1.1
10.41.1.2
10.41.1.3
203.38.201.35
203.38.201.36
203.38.201.37
Gig 0/0 Gig 0/1

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DYNAMICNATCONFIGURATION
R1
PC3
PC2
PC1
10.41.1.1
10.41.1.2
10.41.1.3
ADDRESS POOL
203.38.201.35
203.38.201.36
203.38.201.37
203.38.201.38
Gig 0/0 Gig 0/1

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PATCONFIGURATION
R1
PC3
PC2
PC1
10.41.1.1
10.41.1.2
10.41.1.3
203.38.201.35

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NATFORIPv6
• IPv6 has a huge address space so
there is no need for NAT for
private -> public translation
• Only used for communication
between IPv4 and IPv6 (NAT-64)
NAT 64 Router

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