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Understanding TCP/UDP Port Functions

  • Identify common TCP and UDP default ports.

Each TCP/IP or application has a port associated with it. When a communication is received, the target port number is checked to determine which protocol or service it is destined for. The request is then forwarded to that protocol or service. For example, consider HTTP, whose assigned port number is 80. When a web browser forms a request for a web page, that request is sent to port 80 on the target system. When the target system receives the request, it examines the port number. When it sees that the port is 80, it forwards the request to the web server application.

TCP/IP has 65,535 ports available, with 0 to 1023 labeled as the well-known ports. Although a detailed understanding of the 65,535 ports is not necessary for the Network+ exam, you need to understand the numbers of some well-known ports. Network administration often requires you to specify port assignments when you work with applications and configuring services. Table 3.7 shows some of the most common port assignments.

Table 3.7. TCP/IP Port Assignments for Commonly Used Protocols

Protocol TCP Ports

Port Assignment

FTP

20

FTP

21

SSH

22

Telnet

23

SMTP

25

HTTP

80

POP3

110

NNTP

119

NTP

123

IMAP4

143

HTTPS

443

RDP

3389

UDP Ports

TFTP

69

DNS

53

DHCP (and BOOTP server)

67

DHCP (and BOOTP client)

68

SNMP

161

RDP

3389

Cram Quiz

  1. As the network administrator, you decide to block port 80. Which of the following services will be unavailable for network users?

    • circle.jpg A. DNS
    • circle.jpg B. POP3
    • circle.jpg C. FTP
    • circle.jpg D. HTTP
  2. Which of the following is the most commonly used port for FTP in modern implementations?
    • circle.jpg A. 20
    • circle.jpg B. 21
    • circle.jpg C. 23
    • circle.jpg D. 27

Cram Quiz Answers

  1. D. The HTTP service uses port 80, so blocking port 80 prevents users from using the HTTP service. Answer A is incorrect because DNS uses port 53. Answer B is incorrect because POP3 uses port 110. Answer C is incorrect because FTP uses port 21.
  2. B. The most commonly used port for FTP in modern implementations is 21.
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