CCNA Cisco Certified Network Associate Study Guide - FTP Server

114 Chapter 3 � Internet Protocol
The different port numbers that can be used are explained below:
� Numbers below 1024 are considered well-known port numbers and
are defined in RFC 1700.
� Numbers 1024 and above are used by the upper layers to set up sessions
with other hosts and by TCP to use as source and destination
addresses in the TCP segment.
TCP Session: Source Port
The following listing shows a TCP session captured with the Etherpeek analyzer
software. Notice that the source host makes up the source port, which
in this case is 5972. The destination port is 23, which is used to tell the receiving
host the purpose of the intended connection (Telnet).
TCP - Transport Control Protocol
Source Port: 5973
Destination Port: 23
Sequence Number: 1456389907
Ack Number: 1242056456
Offset: 5
Reserved: %000000
Code: %011000
Ack is valid
Push Request
Window: 61320
Checksum: 0x61a6
Urgent Pointer: 0
No TCP Options
TCP Data Area:
vL.5.+.5.+.5.+.5 76 4c 19 35 11 2b 19 35 11 2b 19 35
11 2b 19 35 +. 11 2b 19
Frame Check Sequence: 0x0d00000f
As you saw in the above TCP session, the source host makes up the source
port. But why is it that the source makes up a port number? The reason is to
differentiate between sessions with different hosts. How else would a server
know where information is coming from if it didn’t have a different number
from a sending host? TCP and the upper layers don’t use hardware and logical