Mezz,
1) The 3rd (network) Layer of the OSI Model is in charge of getting packets from one location to another using various methods and routing protocols to ensure the delivery is done in the best and fastest possible manner.
The 4th (transport) layer of the OSI Model takes care of the 'chopping' of data into small datagrams which are then passed to the 3rd layer to be delivered to their destination. The 4th layer also starts and completes any negotiations with the remote host to which the data is to be delivered.
Each layer has a specific function to ensure the job is done correctly, however some functions might be similar in between these layers, but are only relevant to that particular layer. E.g a checksum provided on layer 3 is not the same as a checksum provided on layer 4. This is because the 'checksum' is unique for each layer as it contains more or less information depending on the layer we are talking about.
When in doubt on what function each layer does, I would suggest you check download and refer to our Ethernet II TCP/IP Frame analysis (
www.firewall.cx/download-s06-opa.php
) which contains all the fields and functions each layer performs.
2) Network cards operate on the 1st and 2nd layer as you noted. While their function is primarily focused on the 2nd layer due to the existence of the 'MAC Address', they are also responsible on meeting all the Layer 1 requirements in order to operate correctly (utp cables, electrical signals e.t.c).
Hope that helps !
