[ntp:questions] Re: uk pool problem

Richard B. Gilbert rgilbert88 at comcast.net
Tue Sep 5 16:41:06 UTC 2006

Danny Mayer wrote:
> David Woolley wrote:
>>For several years now, it has been almost essential that it does respond
>>to client requests from other ports, because of network address translation.
> I hope NAT does not REQUIRE different port numbers.
> Danny
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NAT maps public address + port to (RFC 1918) private address + port.  So 
a system with an RFC 1918 address will send an NTP packet 
from port 123 and the NAT router will map it to port 
xxxxx.  When you reply to port xxxxx the router knows to 
map it to port 123.

So yes, in a sense, NAT does require "different" port numbers.  Speaking 
as one of the many behind a NAT router/firewall it all seems to work, 
however improbable it might seem.

If IP V6 ever gets off the ground, there will be enough addresses to go 
around and this subterfuge will no longer be necessary.  IP V6 does not 
appear to be going anywhere in a hurry though!  About three years ago, 
my then boss (manager of network services) saw me answer "Yes" to the IP 
V6 support question asked by Solaris Installation and screamed "No!". 
I had to explain to him that the box would still speak IP V4 to anyone 
who wanted to talk to it using V4 and could speak IP V6 to anyone who 
wanted to use it.  My little LinkSys Router hasn't a clue about IP V6.
Comcast is IP V4.  IP V6 may be coming but it's by no means here yet!!!

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