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.
When preparing for the Cisco CCNA 200-301 exam, you need to have strong knowledge of the Transport Layer of the OSI model and also about TCP/UDP port numbers and their associated applications. Read below for a CCNA prep tutorial about the Transport Layer.
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. For example, DNS runs on UDP port 53.
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)
SSH (TCP Port 22)
Telnet (TCP port 23)
SMTP email (TCP port 25)
HTTP (TCP port 80)
HTTPs (TCP port 443)
POP3 email (TCP port 110)
DNS (TCP or UDP port 53)
TFTP (UDP port 69)
SNMP (UDP port 161)
RIP (UDP port 520)
You need to learn the mapping between the basic port numbers with their respective application for your CCNA exam. The above port examples are very important.