Summary (12.7)–Cisco Application Layer Services

The following is a summary of each topic in the chapter:

• Network Application Services—Some of the protocols used for network services are DNS, SSH, SMTP, POP, IMAP, DHCP, HTTP, and FTP. Each of these services relies on protocols from the TCP/IP protocol suite to reliably communicate the information between the clients and the servers.
• Domain Name System—DNS provides a way for hosts to use a domain name to request the IP address of a specific server. DNS names are registered and organized on the Internet within specific high-level groups, or domains. Some of the most common high-level domains on the Internet are .com, .edu, and .net.

A DNS server contains a table that associates host names in a domain with corresponding IP addresses. When a client has the name of a server, such as a web server, but needs to find the IP address, it sends a request to the DNS server on port 53. The client uses the IP address of the DNS server configured in the DNS settings of the host IP configuration.

• Web Clients and Servers—When a web client receives the IP address of a web server, the client browser uses that IP address and port 80 to request web services. This request is sent to the server using HTTP.

When the server receives a port 80 request, the server responds to the client request and sends the web page to the client. The information content of a web page is encoded using specialized markup languages such as HTML, XML, and XHTML. The markup language coding tells the browser how to format the web page and what graphics and fonts to use.

• FTP Clients and Servers—FTP lets you transfer files from one computer to another. A host running FTP client software can access an FTP server to perform various file management functions. The FTP server enables a client to exchange files between devices and to manage files remotely by sending file management commands such as delete or rename.

FTP uses two different ports to communicate between client and server. To begin an FTP session, control connection requests are sent to the server using destination TCP port 21. When the session is opened, the server changes to TCP port 20 to transfer the data files.

Most client operating systems such as Windows, macOS, and Linux include a command-line interface for FTP. GUI-based FTP client software provides a drag-and-drop interface for FTP.

• Virtual Terminals—Telnet is a method of emulating text-based terminal devices over the data network. Both the protocol itself and the client software that implements the protocol are commonly referred to as Telnet. Telnet servers listen for client requests on TCP port 23. A connection using Telnet is called a virtual terminal (vty) session, or connection. Rather than using a physical device to connect to the server, Telnet uses software to create a virtual device that provides the same features of a terminal session with access to the server CLI.

After a Telnet connection is established, users can perform any authorized function on the server, just as if they were using a command-line session on the server itself. But Telnet does not support the transport of encrypted data. When security is a concern, SSH provides the structure for secure remote login and other secure network services. It also provides stronger authentication than Telnet and supports the transport of session data using encryption.

• Email and Messaging—Each mail server receives and stores mail for users who have mailboxes configured on the mail server. Each user with a mailbox must then use an email client to access the mail server and read these messages. Many Internet messaging systems use a web-based client to access email. Various application protocols used in processing email include SMTP, POP3, and IMAP4.

An email client uses SMTP to send messages to its local email server. The local server then decides if the message is destined for a local mailbox or if the message is addressed to a mailbox on another server. SMTP requests are sent to port 25.

A server with POP3 receives and stores messages addressed to its users. When the client connects to the email server, the messages are downloaded to the client. The server deletes the messages after they have been accessed by the client. Clients contact POP3 servers on port 110.

Unlike POP, IMAP keeps the messages in the mailboxes on the server, unless they are deleted by the user. The most current version of IMAP is IMAP4, which listens for client requests on port 143.

Text messaging is one of the most popular communication tools in use today. Text messages may also be called instant messages, direct messages, private messages, or chat messages. Text messaging enables users to communicate over the Internet in real time. Text messaging services on a computer are usually accessed through a web-based client. There are also a number of standalone text message clients.

An Internet telephony client uses peer-to-peer technology similar to instant messaging. IP telephony uses VoIP technology, which converts analog voice signals into digital data. The voice data is encapsulated into IP packets that carry the phone call through the network.

Practice

The following activities provide practice with the topics introduced in this chapter.

Labs

Lab—Observe DNS Name Resolution (12.2.4)

Packet Tracer Activities 

Packet Tracer—Observe Web Request (12.3.3)

Packet Tracer—Use FTP Services (12.4.4)

Packet Tracer—Use Telnet and SSH (12.5.4)

Check Your Understanding Questions

Complete all the review questions listed here to test your understanding of the topics and concepts in this chapter. Appendix A, “Answers to the ‘Check Your Understanding‛ Questions,” lists the answers.

1. A new employee is attempting to configure a cell phone to connect to the company‛s email server. Which port number should be selected when using the POP3 protocol to access messages stored on the email server?

1. 69
2. 25
3. 143
4. 110

2. What is the advantage of using SSH over Telnet?

1. SSH provides secure communication to access hosts.
2. SSH supports authentication for a connection request.
3. SSH operates faster than Telnet.
4. SSH is easier to use.

3. Which protocol retains a message in a mailbox on a server, even after the message is accessed by a user on a local client device?

1. IMAP4
2. POP3
3. SMTP
4. DNS

4. A new employee is attempting to configure a cell phone to connect to the company‛s email server. Which port number should be selected when using the IMAP4 protocol to access email messages stored on the local server?

1. 25
2. 69
3. 110
4. 143

5. Which protocol is used to transfer web pages from a server to a client device?

1. HTTP
2. POP3
3. SMTP
4. SSH
5. HTML

6. The sender‛s email server forwards messages to the email server of the recipient. Which port number should be selected when using the SMTP protocol to forward email messages from one SMTP server to another SMTP server?

1. 25
2. 143
3. 23
4. 110

7. Which protocol enables a user to type www.cisco.com instead of an IP address to access the web server?

1. SNMP
2. HTML
3. HTTP
4. DNS
5. FTP

8. Which communication tool enables multiple users to communicate with each other in real time by using a smartphone application or social media site?

1. Instant messaging
2. Web mail
3. Blog
4. Email

9. An employee of a company is attempting to remotely access a computer system using a virtual terminal (vty) session via Telnet. Which destination port number should the employee put in the remote access software being used?

1. 20
2. 110
3. 23
4. 69

10. What characteristics describe an FTP connection? (Choose two.)

1. A large file requires more than two connections between the client and server to successfully download it.
2. The server established the first connection with the client to control traffic that consists of server commands and client replies.
3. Files can be downloaded from or uploaded to the server.
4. The first connection established is for traffic control, and the second connection is created to transfer a file.
5. The client needs to run a daemon program to establish an FTP connection with a server.

11. Which protocol do web servers use to serve up a web page?

1. FTP
2. POP3
3. HTTP
4. IMAP4

12. Which protocol removes messages from the server after they have been accessed by a client?

1. POP3
2. DNS
3. IMAP4
4. SMTP