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»Cisco Forum FAQ »NAT, PAT, Port Forward, Internet and Server Access: Introduction and Practices
Network Address Translation or NAT is an Internet standard (IETF RFC 1631) that enables a local-area network (LAN) to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. A NAT box (e.g. a router) located where the LAN meets the Internet makes all necessary IP address translations.
- Hides internal IP addresses.
- Enables a company to use more internal IP addresses. Since they're used internally only, there's no possibility of conflict with IP addresses used by other companies and organisations.
The links below provide NAT information in greater detail:
An introduction to NAT
NAT FAQ @ Cisco.com
How to configure NAT?
NAT and route maps.
NAT Implementation: Sample Configurations
1. Basic Internet Access (Outbound Traffic Only) - No Public Servers
Various Cisco Router, PIX/ASA NAT/PAT Sample Configurations
2. Basic Internet Access and Public Servers (Inbound and Outbound Traffic)
Running Servers using Cisco Router, PIX/ASA NAT/PAT Technology
Generic NAT configuration
This configuration was worked up on a cisco 831 with 12.2(13)ZH2 software but should work on any cisco router with a modern version of IOS, just adjust the interfaces accordingly.
no service pad
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
logging queue-limit 100
ip dhcp excluded-address 192.168.4.1 192.168.4.10
ip dhcp pool LOCALPOOL
network 192.168.4.0 255.255.255.0
ip audit notify log
ip audit po max-events 100
no ftp-server write-enable
description Inside private interface
ip address 192.168.4.1 255.255.255.0
ip nat inside
hold-queue 100 out
description Outside public interface
ip address dhcp
ip nat outside
ip nat inside source list 1 interface Ethernet1 overload
ip http server
no ip http secure-server
access-list 1 permit 192.168.4.0 0.0.0.255
line con 0
no modem enable
line aux 0
line vty 0 4
scheduler max-task-time 5000
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