need to fully understand 'show ip nat translation' table

[meng.li3]

Dear all,

these days I spent a lots of efforts to try to understand NAT. it takes me sometimes to understand the subtility bt 'ip nat inside source', 'ip nat outside source' and 'ip nat inside destination' etc..

what i want to do further is to understand the 'show ip nat translation' table. when the new entry get added, whethere it is temporiry or perminent, what does it mean by 'protocol' field, when all the four address field have values and when only two of the field have value etc....

It seems all the doc on the internet is only explaining different types of NAT and the definition of 'inside local', 'inside global', 'outside local' and 'outside global'.

if anyone can recommend some doc explain the details of 'show ip nat translation' table, it will truely help me. thanks.

Li Meng

[Neeraj Arora]

Li,

you've asked a very subjective question, I'll try to answer pointwise:

Q: Whether a NAT entry is temporary or permanent?

A: Only Static NAT configured remains Permanently in the NAT translation table, rest all other entries are temporary and timeout after a specific idle time (different transport protocol has different timeout periods)

Q: what does it mean by 'protocol' field

A: Generally its either TCP, UDP, ICMP, ESP

Q: when all the four address field have values and when only two of the field have value etc....

A: When only two fields have value, that means its a Static NAT config and is a permanent entry. Packets matching this static rule will use it as a reference and will create a dynamic (temporary) translation entry based on it with unique source/destination port number. This dynamic translation entry will have all 4 fields filled up with some values (ip address & port), this translation entry is the active entry which a return packet will also use for Reverse translation

Now I do not know any good documents related to NAT translation explanation but here are couple of links that might prove useful:

Network Address Translation (NAT) FAQ

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_q_and_a_item09186a00800e523b.shtml

Basic rule of Network Address Translation (NAT) – Routers

http://blog.instruosolutions.com/2012/01/08/very-basic-rule-of-network-address-translation-nat-routers-switches/

hope it helps

Neeraj

[meng.li3]

Hi Neeraj,

thanks for your information!

the URL is not helpfull but your point-to-point answer really help me, thanks again!

Li Meng