The Transmission Control Protocol/Internet Protocol (TCP/IP) study guide

Build the communication chain

When explaining a networked application, begin with the application requirement. For example, a browser needs a web page. The browser acts as the client and sends a request to a web server. The server responds by returning the web page in text form. The browser also retrieves the required page resources and renders them.

Then connect this exchange to the TCP/IP stack. At the application layer, HTTP or HTTPS defines the web communication. At the transport layer, the client makes a TCP connection to the server's port. A socket identifies an endpoint of the communication, and the client also uses a client port. At the network layer, IP addresses identify the network and host parts involved in delivery. At the link layer, MAC addresses are relevant to communication over the local network.

A strong explanation distinguishes the roles rather than listing terms. HTTP is not an IP addressing system, and a port is not an IP address. The IP address identifies network and host information; the port distinguishes an application endpoint; the socket represents a communication endpoint. A well-known port is associated with a standard service, while a client port is used by the client side.

Apply protocols to scenarios

For file transfer, identify the FTP client and FTP server and state whether access is anonymous or non-anonymous. For email, explain that SMTP is used when sending and POP3 when retrieving. For remote administration, explain that an SSH client makes a TCP connection to a remote port, logs in securely and sends commands to execute on the remote computer. If a question mentions application-level commands, relate them to the relevant protocol: GET for HTTP, SMTP commands for sending email, or POP3 for retrieving email.

Reason about addressing and access

When a device is on a private network, distinguish its non-routable private IP address from a routable public IP address. DHCP supplies the system for configuring devices. NAT provides translation between private and public addressing. If an external connection must reach an internal service, port forwarding directs communication arriving at a public address and port towards the relevant internal device or service. Subnetting should be explained through the network identifier, host identifier and subnet mask: the mask identifies which part of the IP address is the network identifier.

IPv4 and IPv6 are the two IP standards named in the specification. IPv6 was introduced because a newer address standard was needed. Avoid claiming that every network uses only one standard unless the question supplies that information.

Explain modern web applications

Start with the client-server model: request from client, response from server. For a database-connected browser application, REST relies on HTTP request methods. JavaScript running in the browser calls a REST API created and run on the server. JSON or XML can carry the data. Map the method carefully: GET to SELECT and retrieve, POST to INSERT and create, PUT to UPDATE, and DELETE to DELETE. When comparing JSON with XML, give the specified distinctions: JSON is easier to read, more compact, easier to create and easier for computers to parse.

WebSockets require a different explanation from ordinary request-response communication. They establish a persistent, full-duplex socket connection between browser and server over TCP. Because both parties can send data at any time, WebSockets suit communication that needs an ongoing connection.

Self-check questions

1. Can you describe the role of every TCP/IP layer without confusing a layer with a protocol?
2. Can you distinguish a well-known port, a client port, a socket, an IP address and a MAC address?
3. Can you select FTP, HTTP, HTTPS, SMTP, POP3 or SSH for a stated task and justify the choice?
4. Can you trace a browser request from client to server and explain the response and rendering?
5. Can you explain how subnet masks identify the network part of an IP address?
6. Can you distinguish public and private addressing and explain the purposes of DHCP, NAT and port forwarding?
7. Can you map CRUD operations to REST methods and database functions without swapping POST, PUT, GET and DELETE?
8. Can you compare thin-client and thick-client computing by explaining where processing occurs?

In an exam answer, use a sequence of cause and effect: identify the device or application, state the protocol, explain the connection or addressing involved, and then describe the resulting request, response or service.