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Beginner

PAT overloading

Hi Folks, 

 

Based on Cisco documentation about NAT, the following ports are used during the overloading process.

 

 When configuring for PAT (overloading), what is the maximum number of translations that can be created per inside global IP address?

 

A. PAT (overloading) divides the available ports per global IP address into three ranges: 0-511, 512-1023, and 1024-65535. PAT assigns a unique source port for each UDP or TCP session. It attempts to assign the same port value of the original request, but if the original source port has already been used, it starts scanning from the beginning of the particular port range to find the first available port and assigns it to the conversation. There is an exception for 12.2S code base. 12.2S code base uses different port logic, and there is no port reservation.

 

Let's say there are 10 tcp/udp applications and there are also 10 ports assigned during the PAT process. 

When this applications stop and the NAT timeouts, do the number of available ports for translation reverts back  to the original number as before? 

I am thinking if this is not the case, then there will come a time when all available ports are used up, then NAT exhaustion happens....

 

Is there a way to ensure , this does not happen?

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Hall of Fame Master

Hello @markmark1983 ,

when NAT entries are aged out and removed from NAT table the involved ports are made free for use, otherwise PAT would not be a valid solution to save on public address space.

 

Hope to help

Giuseppe

 

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Hall of Fame Master

Hello @markmark1983 ,

when NAT entries are aged out and removed from NAT table the involved ports are made free for use, otherwise PAT would not be a valid solution to save on public address space.

 

Hope to help

Giuseppe

 

View solution in original post

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thank you @Giuseppe Larosa 

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VIP Expert

Hello,

 

on a side note, I think ever since time, the NAT tcp translation timeout has been 86,400 seconds (24 hours). I wonder if that default value still makes sense in today's networking environment....after all, every entry uses up (a small amount of) memory, but still, why the 24 hours ?