tailieunhanh - Networking: A Beginner’s Guide Fifth Edition- P23

Networking: A Beginner’s Guide Fifth Edition- P23:I have run into many people over the years who have gained good even impressive working knowledge of PCs, operating systems, applications, and common problems and solutions. Many of these people are wizards with desktop computers. | 92 Networking A Beginner s Guide tftp 69 udp Trivial File Transfer gopher 70 tcp finger 79 tcp http 80 tcp www www-http World Wide Web kerberos- sec 88 tcp krb5 Kerberos kerberos- sec 88 udp krb5 Kerberos rtelnet 107 tcp Remote Telnet Service pop2 109 tcp postoffice POP-V2 pop3 110 tcp POP v3- nntp 119 tcp usenet NNTP ntp 123 udp Network Time Protocol snmp 161 udp SNMP snmptrap 162 udp snmp-trap SNMP trap print-srv 170 tcp Network PostScript irc 194 tcp Relay Chat Prot ipx 213 udp IPX over IP ldap 389 tcp Lightweight DAP https 443 tcp MCom https 443 udp MCom who 513 udp whod cmd 514 tcp shell syslog 514 udp printer 515 tcp spooler router 520 udp route routed netnews 532 tcp readnews uucp 540 tcp uucpd wins 1512 tcp Windows Name Service As you can see most of the Internet services that you might be familiar with actually work through the use of TCP and or UDP ports such as HTTP for the Web SMTP for e-mail NNTP for Usenet and so forth. The use of ports ensures that network communications intended for a particular purpose are not confused with others that might also be arriving at the same machine. Ports allow the receiving machine to direct arriving data appropriately. An example is a server that hosts web pages and also receives and processes e-mail. Packets arriving at port 80 will be sent to the web-serving software while those that arrive at port 25 will go to the e-mail software. Other services on the machine such as Telnet and FTP can also function concurrently through this mechanism. IP Packets and IP Addressing IP packets include addresses that uniquely define every computer connected to the Internet see Figure 8-1 . These addresses are used to route packets from a sending node to a receiving node. Because all the routers on the Internet know the network addresses to which they are connected they can accurately forward packets destined for a remote network. Chapter 8 Understanding Networking Protocols 93 Version 4 bits Header length 4 bits Type of service 8 .

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