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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.