IP address disappearing and reappearing in TCP/IP

[semsaudade]

Hi to everybody. I am trying to understand a feature of TCP/IP layer model. Let's suppose we have a browser requesting a page from a server. It receives an URL from the user, converts it in a IP address (with the help of a DNS server) and then it sends its HTTP request to the destination IP.

Up to this point, it is quite clear. My problem is to understand what's happening with such destination IP. I mean, the browser passes its message down to TCP layer and in this layer there is no IP, just destination and source port numbers. So, apparently, IP addresses disappeared from our message.

Then TCP layer passes its segment down to IP layer and here IP address appears once again! What's happened? Is there here a breach to the general rule stating that each layer comunicates just with its adiacent layers? How IP address can go down from the top layer (application layer in TCP/IP model) to IP layer (network layer) apparently without passing through TCP (transport layer)?

Thanks to everybody for helping!

 

[bill001g]

You need to run a wireshark capture and watch traffic going back and forth. Trying to memorize the layer stuff with no concept of what is really going on will get you no where.

The traffic does not actually pass through layers the software representing the layers only looks at its part of the data it cares. The other data used by other layers may or may not exist in the data stream. A router for example will only look for IP addresses it does not care if there are HTTP headers or skype messages it just ignores everything else except the narrow part of the data it is designed to look at.