I've got one of those problems that suddenly makes me realize how little I understand about the gory details of networking. Any help about the following would be greatly appreciated.

I'm running a "relatively standard" home network environment. That is, I've got a DSL connection to my ISP, and am running a router/NAT environment that connects a number of machines -- Mac, Windows, and Linux -- to the external network. Internally, all the machines have addresses in the range of 192.168.0.x. However, because of the static/NAT address mapping, the machines whose internal addresses are mapped to external static addresses show up at external hosts as 216.xxx.yyy.zzz (the address range assigned to me by my ISP). The other machines, which are on the network via NAT, show up as 66.xxx.yyy.zzz, the address assigned by my ISP. Finally, I'm running a local DNS server that handles addressing issues for my domains and sub-domains that are hosted on local machines. All the machines use this local server for their DNS, with my ISP's DNS server as secondary. All of this is to say that it's more or less a pretty typical configuration, and, in general, everything is working fine.

Here's where it gets tricky:

* Machine A is a Fedora 10 linux box, which hosts an externally visible domain and runs, among other things, a web server. It's assigned to one of the addresses that's mapped to a static IP address.

* Machine B is a MacBook Pro, running Snow Leopard (10.6). It typically runs at 192.168.0.6, which is NOT one of the addresses mapped to a static address, but "just" a NAT address.

Now, suppose Machine B hits the web browser run by Machine A: the log files should show the traffic arriving from the local address, 192.168.0.6. However, it instead shows up as coming from 66.xxx.yyy.zzz, the external NAT-affiliated address. This seems wrong, especially since all the other machines on my network show 192.168-type addresses when hitting the web server.
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I now change Machine B's address to 192.168.0.5 -- one of the statically-mapped addresses. Browsing traffic is now arriving from 192.168.0.5, which is (I think) as it should be.

I now change Machine B's address back to 192.168.0.6. The web traffic on Machine B is now reported to be arriving from 192.168.0.6 -- NOT 66.xxx.yyy.zzz.

Thus my confusion. It's complicated by the fact that I don't know where to look for the problem. All the log files on all the machines seem relatively normal; there are least no glaring problems screaming out from them. I'm tempted to blame the MacBook, since it's the only one showing the problem, but I'd feel better if I knew what to do and how to fix it.

Any advice out there? This is driving me kinda nuts. Thanks!

In answer to your question: Machine B, the macbook, can get to the machine A web server either internally, or by going out to the internet and coming back in. Your internal DNS server may have something to do with this. Routers are DNS servers, but you also have another internal DNS server. The other internal DNS server may be sending machine B to the internet, and then back to your web server. There could also be a conflict with the 192.168.0.6 address.