My CISCO Journal

In this part of NAT we will discuss the last common type of NAT, which is Static NAT. we will also discuss some of the features and special services on NAT. The discussion ends with a brief overview of SNAT (state-full Network Address Translation)

Static NAT:

Now let’s suppose we have our email server on 192.168.4.1/24 on R1. Suppose we want to create a static NAT entry to for SMTP (simple mail transfer protocol) on this server, so that it can communicate with the outside world and its SMTP service should be accessible from outside as well.

 It is achieved by enabling Static NAT on R2; we just need to issue the following command in global configuration mode:

ip nat inside source static tcp 192.168.4.1 25 interface Serial1/0 25

Port 25 = SMTP port or email server port

On Port 25 if any packet is received on our Router R2 interface serial 1/0, it will be forwarded to our SMPT server 192.168.4.1 and vice versa.  So if we see our NAT translations, we will see a permanent static NAT entry for TCP port 25 in it:

 

This entry will not expire from the NAT table after translation timeout.  All dynamic entries are deleted from the NAT translation table after translation timeout. As we have already discussed, static mapping is mostly use for server accessibility from outside the local network. 

Please remember, STATIC NAT is both way NAT i.e. we can use it for accessibility from outside our network.

Troubleshooting NAT:

Some of the points we need to keep in mind while troubleshooting NAT:

 -- to be continued --