CCNA Exam Prep: Understanding the Transport Layer and Port Numbers Tutorial.

This is another CCNA exam preparation tutorial about the Transport Layer of the TCP/IP suite. Understanding the purpose and functionality of the Transport Layer (Layer 4 in the OSI model) is fundamental for understanding how data is transmitted in network environments.

The Transport Layer resides between Application and Network layers and has the critical role of providing communication services directly to the application processes running on different hosts. As message data arrives from an Application Layer to the Transport Layer, the latter places a header on the data in order to identify from which application the data was received. This Transport Layer header contains a Source Port Number and a Destination Port Number. The Port Numbers identify the application from which the data was received or is destined to. Read more on port numbers later in this article.

Another role of the Transport Layer is to provide reliable communication and flow control. The two most well known protocols that work in the Transport Layer are TCP and UDP. TCP is a connection-oriented reliable protocol that uses flow control mechanisms and acknowledged data delivery to offer reliable communication. UDP on the other hand is a connectionless and unacknowledged protocol.

Now, the Network Layer (IP layer 3), which is below the Transport Layer and is responsible to route packets between hosts, does not offer reliability for data delivery. That is why we use TCP on top of IP (hence the TCP/IP suite) in order to provide reliable and error free data flow communication.

As we mentioned above, the Transport Layer uses Port Numbers to differentiate between various applications that might need communication services. You should be ready to get questions in the CCNA exam regarding port numbers and their association with different applications. You should know that port numbers below 1024 are reserved for several well-known applications. Some examples are shown below:

FTP (TCP Port 21)
Telnet (TCP port 23)
SMTP email (TCP port 25)
POP3 email (TCP port 110)
DNS (TCP or UDP port 53)
TFTP (UDP port 69)
SNMP (UDP port 161)
RIP (UDP port 520)

Harris Andrea is a Cisco Certified Professional (CCNA, CCNP, CCSP) with more than 10 years experience in the networking field. He is currently employed as a senior network engineer in a leading ISP company.

Harris recommends the following CCNA Training package to pass your CCNA exam Guaranteed (You can watch also a Demo Training Video).

CCNA Video Training
CCNA Video Training