PAT - Maximum number of translations

Unanswered Question
Aug 23rd, 2007

I don't know the maximum of translation slots available when PAT is used

1) PAT uses the high ports for translations; therefore the available translation slots per IP are calculated

via 65536-1024 = 64512.

2) Almost unlimited as a hash value is used to identify the translation instead of using the TCP port. The

hash value is calculated using the source port and IP address as well as destination port and IP address,

this will allow to have more that 65000 connection with one IP.

I have this problem too.
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rajbhatt Fri, 08/24/2007 - 02:18

Hi ,

Please check this out :

Port Address Translation (PAT) maps a single global IP address to many local addresses. PAT extends the range of available outside addresses at your site by dynamically assigning unique port numbers to the outside address as a connection is requested. A single IP addresses has up to 65,535 ports that are available for making connections. For PAT, the port number uniquely identifies each connection.

When we add a global pool the last global command should always be a pat address in case the pool is exhausted

Raj

hsajwan Fri, 08/24/2007 - 05:21

I had replied to this message yesterday but looks like, it got lost somehow..anyways, here's my answer again:

You second point above (which talks about using hash value) is wrong. The first one is better and correct but let me give you some more details about this whole process:

The firewall, when translating port for NAT overload (or PAT), splits the available ports into three pools:

Low: 0-511

Mid: 512-1023

High: 1024-65535

If a packet inside you network comes into the Firewall destined for the Internet, and if

source port falls into one of those pools, the PIX will translate it to another port in

that pool. When the Firewall first starts translating addresses, it starts with the lowest port number in each pool. That means the first packet sourced internally from a high port will get sent on the Internet with a new source port of 1024. The next high port translation will go out with a source port of 1025, so on and so forth.

Here's a link where you can find more about this:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_q_and_a_item09186a00800e523b.shtml#qa13

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