Networking.docx

Host-to-Host Layer Protocols After the myriad protocols at the Process/Application layer, the simplicity of the Host-to-Host layer is welcome. At this layer there are two alternatives within the TCP/IP suite: TCP and UDP. The major difference between the two is that TCP guarantees packet delivery th...

Host-to-Host Layer Protocols After the myriad protocols at the Process/Application layer, the simplicity of the Host-to-Host layer is welcome. At this layer there are two alternatives within the TCP/IP suite: TCP and UDP. The major difference between the two is that TCP guarantees packet delivery through the use of a virtual circuit and data acknowledgements and UDP does not. Because374of this, TCP is often referred to as connection-oriented, whereas UDP is connectionless. Because UDP is connectionless, it does tend to be somewhat faster, but we’re talking about milliseconds here. Another key concept to understand about TCP and UDP is the use of port numbers. Imagine a web server that is managing connections from incoming users who are viewing web content and others who are downloading files. TCP and UDP use port numbers to keep track of these conversations and make sure that the data gets to the right application and right end user. Conversely, when a client makes a request of a server, it needs to do so on a specific port to make sure that the right application on the server hears the request. For example, web servers are listening for HTTP requests on port 80, so web browsers need to make their requests on that port. A good analogy for understanding port numbers is to think of cable or satellite television. In this analogy, the IP address is your house. The cable company needs to know where to send the data. But once the data is in your house, which channel are you going to receive it on? If you want sports, that might be on one channel, but weather is on a different channel, and the cooking show is on yet another. Those channels are analogous to ports. You know that if you want a cooking show, you need to turn to channel 923 (or whatever). Similarly, the client computer on a network knows that if it needs to ask a question in HTTP, it needs to do it on port 80. There are 65,536 ports, numbered from 0 to 65535. Ports 0 through 1023 are called the well-known ports and are assigned to commonly used services, and 1024 through 49151 are called the registered ports. Anything from 49152 to 65535 is free to be used by application vendors. Fortunately, you don’t need to memorize them all.  TCP/IP applications combine the host’s IP address with the port number in order to communicate. This combination is known as a socket. Table 7.1 shows the ports used by some of the more common protocols. You should know each of these for the A+ exam. Table 7.1 Common port numbers Service Protocol Port(s) FTP TCP 20, 21 SSH TCP 22 Telnet TCP 23 SMTP TCP 25 DNS TCP/UDP 53 DHCP UDP 67, 68 HTTP TCP 80 POP3 TCP 110 NetBIOS/NetBT TCP 137–139 IMAP4 TCP 143 SNMP UDP 161, 162 LDAP TCP 389 SLP TCP 427 HTTPS TCP 443 SMB/CIFS TCP 445 AFP TCP 548 RDP TCP 3389 375 A complete list of registered port numbers can be found at www.iana.org and several other sites, such as Wikipedia.