[mythtv-users] Re: Why only numeric IP addresses work?

Joseph A. Caputo jcaputo1 at comcast.net
Wed Oct 13 19:49:29 UTC 2004

On Wednesday 13 October 2004 15:01, Mitko Haralanov wrote:
> On Wed, 13 Oct 2004 09:30:30 -0400
> "Joseph A. Caputo" <jcaputo1 at comcast.net> wrote:
> > Cool... I never thought of using DynDNS as my own personal
> > 'internal' DNS server.  I'd just assumed that they'd disallow
> > RFC1918 addresses. I guess what they say about "assume" is true...
> > :-)
> Now, I am not an expert but I just don't see how this would work.
> 192.168.x.x addresses are for private networks. Therefore, I can have
> a address, as well as any other person that has a
> 192.168.x.x networks.
> In this case, how would DynDNS know which one of the thouzand
> addresses it should route to?
> As far as I know, as designed the 192.168.x.x (and such) addresses
> are not to be publicly routable...
> But I could be wrong!! :)

DISCLAIMER:  The following is a very high-level description DNS & 
routing; none of this should be considered gospel, nor should I in any 
way be mistaken for a networking expert.  The purpose is only to 
illustrate why DynDNS doesn't care where your packets should be routed.

DynDNS doesn't do *routing*.  It provides DNS (Dynamic Name Services).  
On a very simple level (they have other services, too) all it does is 
map a hostname to an IP address.  If I query my primary DNS server for 
the IP address assigned to the hostname "foobar.dyndns.org", it will 
query the master DNS server for the dyndns.org domain, which will tell 
it the IP address is, say,  My primary DNS server will 
then return that address to the application on my box that requested 
it, at which point that application can continue whatever network 
operation it wants to do.  All packets going out of my box will have a 
destination IP address (or possibly a multicast or broadcast address, 
but NOT a hostname) in the header.  When it hits my router, my router 
will know that is an internal address and route the packet 

So, while it may not be proper netiquette to register a 
globally-resolvable host/domain name with an internal IP address, there 
is no technical reason it can't be done if your main objective is to 
provide yourself with DNS services for your internal network without 
going to the trouble of setting up your own DNS server.

Of course, the drawback to this approach is that if your Internet 
connection ever goes away, or if DynDNS has an interruption in service 
you won't be able to resolve any hostnames in your internal network...


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