Understanding TCP/UDP Port Functions

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
POP3 110
NNTP 119
NTP 123
IMAP4 143
RDP 3389
UDP Ports
DNS 53
DHCP (and BOOTP server) 67
DHCP (and BOOTP client) 68
SNMP 161
RDP 3389