PUTTING IT ALL TOGETHER

So, what happens behind the scenes when you make a connection across the Internet? Let's take the most common scenario of all, a Web page download.

Your PC sends UDP packets to your local DNS server to translate the domain name into an IP address and returns the results. If there's a problem, the DNS lookup will time out, and the browser will complain that the server was not found.

The destination IP address is inserted into the header of your packets, along with your IP address and the port number for HTTP (80). Your request is sent over the TCP transport to the destination host to set up a connection. The route taken to the destination is left up to the routers, who read the IP address and hop from router to router to get there. If the packets can't find the destination host, you will likely see a "destination unreachable" error; or if there is a routing loop in the network, the Time-to-Live (TTL) counter will reach zero and the connection attempt will be timed out.

If the routers find the router on which the destination host resides, the packets go through the network interface to enter the host's LAN segment using the destination IP address' subnet mask, if there is one.

The receiving host reads your message and decides whether to respond. To answer your service request, it uses the TCP three-way handshake and windowing techniques, the connection is established, and the requested Web page download is executed under the management of TCP windowing.

In simplified terms, that's what happens when a hyperlink is clicked in a browser.