How does wireless routing work?

I do not understand how packets from remote systems reach my PC
through a wireless router.

On the local network side of the router, my PC is assigned an IP
address in the range 192.168.*.*. Of course, that is a private network

address range. Packets transmitted to an external network cannot
have 192.168.*.* in the source IP address field, and inbound packets
from an external network should not have 192.168.*.* in the destination

IP address field; they should be blocked eventually by some router, or
by our wireless router as a last resort.

But Ethereal shows that outbound packets from my PC do have
192.168.*.* in the source IP field, and inbound packets from external
systems do have 192.168.*.* in the destination IP field.

The external network side of the router has an IP address assigned
dynamically by the cable ISP -- for example, 24.7.*.* assigned by
Comcast. Websites such as www.showmyip.com confirm that packets
from my PC have that address in the source IP field.

So packets leave my PC with source IP address 192.168.*.*, the
wireless router presumably replaces the source IP field with its own
24.7.*.* IP address, and the remote system sends packets back to
24.7.*.*. Both the Comcast cable network and external systems are
oblivious to the existence of the wireless home network.

My question is: then, how does the wireless router know that such
inbound packets should go to my PC instead of another PC on the
same wireless network? What information does the wireless router
have and use to demultiplex inbound traffic -- be it part of an
established TCP session or a TCP SYN, UDP or ICMP packet?

If you can also point to any online explanation as well as your own,
I would appreciate it. My google searches have turned up nothing
so far.

If it matters, my wireless router is a Linksys WRT54G. But I am
hoping for a more general response based on networking architecture.

TIA.

On 14 Jan 2006 19:52:51 -0800, (E-Mail Removed) wrote:


>My question is: then, how does the wireless router know that such
>inbound packets should go to my PC instead of another PC on the
>same wireless network? What information does the wireless router
>have and use to demultiplex inbound traffic -- be it part of an
>established TCP session or a TCP SYN, UDP or ICMP packet?
>
>If you can also point to any online explanation as well as your own,
>I would appreciate it. My google searches have turned up nothing
>so far.
>

from http://en.wikipedia.org/wiki/NAT

In a typical configuration, a local network uses one of the designated
"private" IP address subnets (such as 192.168.x.x or 10.x.x.x), and a
router on that network has a private address (such as 192.168.0.1) in
that address space. The router is also connected to the Internet with
a single "public" address (known as "overloaded" NAT) or multiple
"public" addresses assigned by an ISP. As traffic passes from the
local network to the Internet, the source address in each packet is
translated on the fly from the private addresses to the public
address(es). The router tracks basic data about each active connection
(particularly the destination address and port). When a reply returns
to the router, it uses the connection tracking data it stored during
the outbound phase to determine where on the internal network to
forward the reply; the TCP or UDP client port numbers are used to
demultiplex the packets in the case of overloaded NAT, or IP address
and port number when multiple public addresses are available, on
packet return. To a system on the Internet, the router itself appears
to be the source/destination for this traffic.

Dave wrote:
> (E-Mail Removed) wrote:
> >My question is: then, how does the wireless router know that such
> >inbound packets should go to my PC instead of another PC on the
> >same wireless network? What information does the wireless router
> >have and use to demultiplex inbound traffic -- be it part of an
> >established TCP session or a TCP SYN, UDP or ICMP packet?
> [....]
> from http://en.wikipedia.org/wiki/NAT

Thanks for the pointer. I had "invented" NAT in my mind as the only
conceivable solution. But I concluded that that "would not" work --
for all the reasons mentioned in the Wikipedia article. I guess I
understood more than I realized. I will study the NAT RFC and other
articles on NAT.

Thanks again.