Configure TCP/IP - MCSA Windows Server 2012 R2: Installation and Configuration Study Guide - page 390

fIgure 8.2 TCP/IP process

Source Host Destination Host

Application Application Application Application

Application Transport Application Transport

Application Internet Application Internet

Application Link Application Link

Network
Packets

When making a connection, the client chooses a source port for the communication that
is usually in the range 1024-65535 (or sometimes in the range 1-65535). This source port
then communicates with a destination port of 80 or 110 on the server side. Every packet
destined for Internet Explorer has a source port number of 80 in the header, and every
packet destined for Mail has a source port number of 110 in the header.
Table 8.1 describes the most common port numbers (you might need to know these
for the exam). You can visit www.iana.org to get the most current and complete list of
port numbers. It's good to become familiar with specific port numbers because it's a
benefit to be able to determine from memory the ports that, for example, allow or block
specific protocols in a firewall. Allowing only port 80, for instance, does not ensure that
all web traffic will be allowed. You must also allow port 443 for certain secure web
traffic.

Simply because a port is “well known” doesn't mean that a given service
must run on it. It's technically valid to run any service on any port, but
doing so is usually a bad idea. For example, if you chose to run your web
server on TCP port 25, clients would need to type www.example.com:25
to reach your website from most browsers.