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Learn Nat In 5 Minutes

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Learn Nat In 5 Minutes

Learn Nat In 5 Minutes

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  • 1. CCNA4.com To have a public IP-Address costs money. To have a whole public subnet, costs more money. So companys with little money but clever admins can use a technic, which helps them to use less public IP-Addresses, but make it possible for lots of clients to connect to the Internet. This technic is called NAT or Network Address Translation. As the name says, an Network Address (IP-Address) is been translated to another address. Mostly, a private IP-Address (172.16.X.X or 196.168.X.X) is translated in a public IP which the company gets from an ISP and pays for the public IP or IPs. There are some terms which must be understanded before going deeper in that material. Its necessary for CCNA Certification to differ between them. Inside local address A private IP Address, not useable in the Internet. Inside global address A public IP address in the inside network. Outside local address A IP on the outside of the network, as its seen by an inside host. Not necessarily public address. Outside global address A IP address in the outside network, which is a public address. There are three forms of NAT Static NAT ONE private IP is translated to ONE public IP. Allways the same private IP is mapped to the same public IP. Dynamic NAT A private IP is mapped to a public IP, which is from a pool of public IPs. It must not allways be the same privat IP, which is mapped to a specific public IP. This is selected dynamically. Overloading Many private IPs are mapped to one public IP. This is also known as PAT (Port Address Translation). Its a Form of dynamic NAT. A private IP establishes a connection, for example is source port 2353. The pakets come to the router. The router translates the IP to a public IP. The router writes the Information about source IP and source Port into its NAT Table. When the answer Pakets arrive from Internet the router again checks its NAT Table and translates the pakets back to the private IP from where the requesting pakets did origin, depending on the port entry in NAT Table. CCNA4.com
  • 2. CCNA4.com Configuration commands Static NAT router(config)#ip nat inside source static local-ip global-ip router(config)#interface fa0/4 router(config-if)#ip nat inside <<>>interface outside network Dynamic NAT router(config)#ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix- length} router(config)#access-list acl-number permit source-IP [source-wildcard] router(config)#ip nat inside source list acl-number pool name router(config)#interface fa0/4 router(config-if)#ip nat inside router(config-if)#exit router(config)#interface s0 router(config-if)#ip nat outside Overloading router(config)#access-list acl-number permit source-IP source-wildcard router(config)#ip nat inside source list acl-number interface interface overload router(config)#interface fa0/4 router(config-if)#ip nat inside router(config-if)#exit router(config)#interface s0 router(config-if)#ip nat outside Read more CCNA4.com