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TCP sends each of these datagrams to IP. Of course it has to tell IP the Internet address of the computer at the other end. Note that this is all IP is concerned about. It doesn't care about what is in the datagram, or even in the TCP header. IP's job is simply to find a route for the datagram and get it to the other end.
In order to allow gateways or other intermediate systems to forward the datagram, it adds its own header. The main things in this header are the source and destination Internet address (32-bit addresses, like 128.6.4.194), the protocol number, and another checksum. The source Internet address is simply the address of your machine. This is necessary so the other end knows where the datagram came from.
The destination Internet address is the address of the other machine. This is necessary so any gateways in the middle know where you want the datagram to go. The protocol number tells IP at the other end to send the datagram to TCP. Although most IP traffic uses TCP, there are other protocols that can use IP, so you have to tell IP which protocol to send the datagram to.
| Version IHL Type of Service Total Length |
|---|
| Identification Flags Fragment Offset |
| Time to Live Protocol Header Checksum |
| Source Address |
| Destination Address |
| TCP header, then your data ...... |
If we represent the IP header by an "I", your file now looks like this:
IT.... IT.... IT.... IT.... IT.... IT.... IT....
Ofcourse, the header contains some additional fields that have not been discussed.
