Hi

Consider the OSI model for reference to your answers.
When you open a webpage and try accessing a webpage, you intiallt 'ask'/'request' for a specific page. this happens in your application layer, later the presentation layer performs its own activites and passes the data to the next layer which is session layer.
It is in the session layer that port allocation is done. The destination port would be 80 for a http file.
When we say a port , we generally refer to a logical endpoint be it on the souce end or on the destination end. There are arounf 60000+ ports ( logical end points).
When you open multiple windows in your net browser, the computer needs to make sure it sends multiple http files to relevant windows, how does that happen? it happens with help of a source port being tied up with the application.
Then the data is passed down to transport layer and then to network layer .
The IP packet from network layer is passed next to the datalink layer, which does the transportation between immediate two points. for such kind of transportation of dataframes, MAC address is used. MAC address is unique to each computer in lay man terms.
But from a different perspective, MAC address is the address assigned to your NIC which is your network card. when you connect your LAN cable to your computer, you plug in to a RJ 45 physical port. This port is actually present on the NIC.
hence NIC can be said to be in both physical layer as well as in you data link layer.
Once the data is onto the cable , incase you have a ADSL service, the digital data needs to be converted into analog data in case your back bone is a PSTN network. Hence the modem basically does the actaully of modulation and demodulation

Though your questions have not been individually answered, I thought this way would be the best for you to understand what falls in where.

I have a question,since it is already a networking thread so i think its better to ask here..
We usually say in TCP, firstly a connection is established then data gets transferred.I have forgot or may be i never thought what is meaning of connection here ?
Q2. What does it mean when we say we firstly establish a connection ?
Q3. What are the steps involved in establishing connection ?
Q4. What is the advantage of establishing connection ?

hello
connection here means that the source computer initialy check weather the destination host is available or not and if it is available than is it ready to build connection with the source computer
now if the destination host is available acknowledgment packet is send to the destination host for data transfer than the destination host replies to it .this is known as "three way handshake "
..now the advantage of establishing aconnection is that when data is sent to the destination host there are the chances that some data might get lost on the network and the destination host doesnt get the complete data so to solve this problem connection is establish ,and if some data packet is lost in the network destination host can send acknowledgment packet that this number of packet is been not received please send again.......hope this will help u

TCP is a connection-oriented protocol.

What it means is that, before sending any packet, two stations communicating via TCP should first establish a connection. Other transport layer protocols like UDP are not connection oriented.

So, in answer to your questions,

When we say TCP establishes a connection, it means that the sender first sends a synchronization packet, called SYN. The receiver gets this and acknowledges "I got your SYN". This packet is called SYN,ACK.
Finally, the sender says, "I got your acknowledgement". This is the ACK. So, the process of sending "SYN >> SYN,ACK >> ACK " is called establishing a TCP session or a connection.

The advantage of establishing a connection is that you can verify the delivery of the packets to the recipient. If you use connection-less protocols like UDP, the reliability or, the assurance that the packet indeed reached the recipient is provided by the application which uses UDP.
If you want to free the application from this burden of providing reliability, you can provide the reliability in the transport layer using a connection-oriented protocol. That's where TCP is sued..