Cisco NAT for IPv4

Port Forwarding

Port forwarding is a static NAT translation with a specified TCP or UDP port number.

ip nat inside source {static {tcp | udp} local-ip local-port global-ip global-port} [extendable]
interface s0/0/0
  ip nat outside
interface fa0/0
  ip nat inside

• extendable - option applied automatically, allows the user to configure several ambiguous static translations, where ambiguous translations are translations with the same local or global address. It allows the router to extend the translations to more than one port if necessary

NAT overload

A PAT-enabled router maintains a table that consists of a mapping of inside local IP addresses and TCP/UDP port numbers to outside local addresses and TCP/UDP port numbers. When traffic returns to the router from the public network, the router would compare the destination port to the PAT mapping table to determine to which inside host the traffic should be sent.

Static NAT

                                         :
                                         : 
                       Outside           :            Inside  
                                 Gi0/0   :  Gi0/1
  PC1 --- 209.165.200.224/29 ------- [  R1  ] ------- 192.168.4.0/24 ---------- WebServer
  .230                            .225   :   .1                                 .10
                                         :			Inside 192.168.4.10/24
                                         :                     Outside 209.165.200.229
                       INTERNET          :       LOCAL LAN
                                         :

R1(config)# ip nat inside source static 192.168.4.10 209.165.200.229
R1(config)# interface gi0/0
R1(config-if)# ip nat outside
R1(config)# interface gi0/1
R1(config-if)# ip nat inside
R1(config)# ip route 0.0.0.0 0.0.0.0 gi0/0

Verify

Before any translations occur

R1#show ip nat translations
Pro  Inside global     Inside local       Outside local      Outside global
---  209.165.200.229   192.168.4.10       ---                ---

Trigger translation by accessing WebServer http://209.165.200.229 from PC1

R1#show ip nat translations
Pro  Inside global     Inside local       Outside local        Outside global
---  209.165.200.229   192.168.4.10       ---                  ---
tcp 209.165.200.229:80 192.168.4.10:80    209.165.200.230:1079 209.165.200.230:1079
tcp 209.165.200.229:80 192.168.4.10:80    209.165.200.230:1080 209.165.200.230:1080
tcp 209.165.200.229:80 192.168.4.10:80    209.165.200.230:1081 209.165.200.230:1081
tcp 209.165.200.229:80 192.168.4.10:80    209.165.200.230:1082 209.165.200.230:1082

Dynamic NAT

R1(config)# ip ant pool NAT-POOL1 209.165.200.226 209.165.200.240 netmask 255.255.255.224  !defines a pool of public IPv4 addresses 
 R1(config)# access-list 1 permit 192.168.0.0 0.0.255.255   !defines addresses eligible to be translated
 R1(config)# ip nat inside source list 1 pool NAT-POOL1     !binds NAT-POOL1 with ACL1
 R1(config)# interface Gi0/0
 R1(config-if)# ip nat outside
 R1(config)# interface Gi0/1
 R1(config-if)# ip nat inside

Troubleshooting

I had problem with a static nat and suddenly it started working, not sure what was related to but I issued following commands on R1:

R1#clear mac-address-table
R1#dynamic clear arp-cache 

References

• more in Cisco_weird_interfaces#NVI_-_NAT_Virtual_Interface
• Static NAT Packet Tracer example