Windows Vista includes a tool called the Windows Operating System
When an application like FTP gets called upon, the application moves down the layers, and TCP is
retrieved. TCP then connects itself to the IP protocol and gets released onto the net- work
through the Link layer (see Figure 2.2). This is a connection-oriented protocol (because TCP is
the protocol that guarantees delivery).
When an application like TFTP gets called, the application moves down the ccna exam
layers, and UDP is retrieved. UDP then connects itself to the IP protocol and gets released onto
the net- work through the Link layer. This is a connectionless protocol (because the UDP protocol
does not have guaranteed delivery).
TCP and UDP rely on port numbers assigned by the Internet Assigned Numbers Authority (IANA) to
forward packets to the appropriate application process. Port numbers are 16-bit integers that are
part of a message header. They identify the application software process that the packet should
be associated with.
For example, let’s say that a client has a copy of Internet Explorer and a copy of Outlook
Express open at the same time. Both applications are sending TCP requests across the Internet to
retrieve web pages and email, respectively. How does the computer know which return packets to
forward to Internet Explorer and which packets to forward to Outlook Express?
When making a connection, the client chooses a source port for the communication that is usually
in the range 1024–65535 (sometimes in the range 1–65535). This source port then communicates
with a mcse 2008
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 Outlook Express has a
source port number of 110 in the header.
Table 2.1 lists 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 full list of port numbers. It’s good to become
familiar with specific port numbers, because it’s an asset 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 in order to reach your website from most browsers.
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