cisco ccna notes

cisco ccna notes.An example of TCP/IP model encapsulation is shown in Figure 1-2.Figure 1-2 shows the TCP/IP stack of a host (for example, Host A) that is trying to open a Web page over HTTP. Here's a step-by-step look at how Host A's TCP/IP stack performs encapsulation. Step 1. In this example, Host A makes a request to open a web page via the HTTP application layer protocol. The HTTP application installed on Host A generates the information (tabbed as HTTP "data" in Figure 1-2). Step 2. The TCP/IP implementation of Host A detects that HTTP relies on the transport layer protocol TCP and sends the HTTP data to the transport layer for further processing. The transport layer protocol TCP will first give the number8 Chapter 1 Network Protocols and Network Device Basicsdata with a TCP header, which will contain information such as the service port number (TCP port number 80 for the request to open a web page), and then sends the data to the next layer (the Internet layer) for further processing. The data encapsulated in the TCP header (net load) constitutes the TCP message segment.After receiving the TCP segment, the Internet layer of the host adds an IP packet header to it, encapsulating it into an IP packet. The IP packet header contains various information about the processing of the IP packet by Host A at the Internet layer, for example, the source and destination IP addresses.Step 4. The IP packet is passed to the link layer for further processing. The TCP/IP stack of Host A finds, after detection, that the IP packet needs to be delivered to the next device as an Ethernet frame over the Ethernet network. Host A adds an Ethernet frame header and tail to the IP packet, and then passes the assembled Ethernet frame to its physical network card (NIC), which is responsible for the transmission of the frame over the physical link.When HTTP data is received encapsulated in an Ethernet frame, the destination receiving host will start processing it from the bottom of the TCP/IP protocol stack. The link layer of the destination host reads and strips the header and frame part of the Ethernet frame before passing the IP packet to the Internet layer for further processing.The Intemet layer of the destination host performs similar processing, passing the TCP message segment to the transport layer, which processes and strips the TCP header and passes the HTTP data to the HTTP application for final processing.2. Network communication based on the TCP/IP model Returning to the previous web browsing example, let's see how a host uses the TCP/IP model to send and receive information over the network connection path.A networkingdevice is a device that implements the TCP/IP model. The TCP/IP model can be either fully implemented in a networking device (a user computer or server is a networking device that fully implements the TCP/IP model) or partially implemented (a router implements the TCP/IP stack up to the Internet layer only).