I know just enough networking to be dangerous. The nitty gritty low level details of NAT are not something I am particularly knowledgeable about.
I accidentally found myself in a discussion earlier today about placing a bunch of our nodes behind a NAT Gateway. (1 public IP address and X private LAN addresses). I called up the 16 bit limit to source and destination port fields in the TCP protocol, (http://www.ietf.org/rfc/rfc793.txt - page 15) and mentioned that it would limit us to some 65,000 connections (65536). -- I am not so confident about that answer anymore. Can you help me with some details?
I understand that an incoming port (server port) on our side can accept as many connections as there are sourceIP x SourcePort combinations. Let's discount those for the time being and focus on connections originating in the LAN, traveling through the NAT Gateway, and ending on a random host at a random port.
On a normal [Linux] system, outgoing connections I believe are limited to 1 per port per Source IP. If we pretend that we live in a simple world where each system only has 1 IP address, then a 'normal system' would be limited to an absolute maximum of 65536 connections.
1) In TCP is a single source IP limited to 65536 MAX theoretical outgoing connections?
2) Or is the limit actually 65536 connections for each Remote Host?
2) [Written another way]: Can the same source port be used for a different remoteHostIP:RemotePort combination?
For example: (Is the following OK?)
Source IP |Source Port |Remote IP|Remote Port 192.168.0.20:36500 --> 126.96.36.199:23 192.168.0.20:36500 --> 188.8.131.52:23
3) Are the answers to questions 1 and 2 different for a ...'not normal system' [Cisco router acting as a NAT Gateway]?
Ex: A specialized networking device that has one public facing IP and up to ~65,000 Lan IPs [or more] behind it? Is there magic at place or is the answer to question 2 just always: yes? (or no)
4) The above questions all assume a stateful TCP connection. Is the story any different with a stateless conection like UDP?
5) Will our LAN be limited to 65536 (or some other theoretical limit) concurrent connections to the outside world through a single public IP address?
Thank you! :)
For purposes of this question, we are behind very BEEFY AND BRAND NEW Cisco Nexus gear (7000 series I think). It may be better to ignore memory/etc limitations unless they can be specifically quantified.