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D.1 Network Basics
This section briefly introduces the function of the TCP and IP
protocols, and explains how IP addresses and ports are used for
communication on the Internet.
D.1.1 TCP/IP
The
Transmission Control Protocol (TCP) and the Internet Protocol (IP)
manage the sending and receiving of messages over the Internet.The Web is a network application that uses the services of TCP and
IP. When a web browser requests a page from a web server, the TCP/IP
services provide a virtual connectiona virtual
circuitbetween the two communicating systems. (The connection
is virtual because the Internet doesn't operate like
an old telephone network. It doesn't create an
actual circuit dedicated to a particular call.)Once a connection is established and acknowledged, the two systems
can communicate by sending messages. These messages can be large,
such as the binary representation of an image, and TCP may fragment
the data into a series of IP datagrams. An IP
datagram is like a postage envelope: it holds all or part of a
message and it's labeled with a destination address
and other fields that manage its transmission from the sender to the
receiver.Each node in the network runs IP software, and the software moves the
datagrams through the network. When an IP node receives a datagram,
it inspects the address and other header fields, looks up a table of
routing information, and sends it on to the next node. Often these
nodes are dedicated routerssystems that form interconnections
between networksbut the nodes can also include standard
machines. IP datagrams are totally independent of each other: the IP
software just moves them from node to node through a network.TCP software performs the function of gluing the fragments in IP
datagrams together at the destination. For example, if a large image
is broken into ten parts that are each stored in a datagram, then TCP
reassembles those parts into a whole. TCP also makes sure the process
is robust: if an IP datagram goes missing, a datagram is corrupted,
duplicate datagrams arrive, or datagrams arrive out of order, then
TCP looks after re-requesting, throwing away duplicates or erroneous
data, or sorting out ordering.
D.1.2 IP Addresses
To
allow communication over heterogeneous networks, each with its own
addressing standard, every location in a network needs a globally
unique IP address. A computer that is connected to the Internet needs
at least one IP address; a node that interconnects two networks needs
two.IP (version 4) addresses are 32-bit numbers that are usually
represented as a series of four decimal numbers between 0 and 255,
separated by a period. An example IP address is 134.148.250.28. Some
IP addresses have special meanings. For example, the IP addresses
127.0.0.0 and 127.0.0.1 are reserved for loopback testing on a host.
If a connection is to be made from a client to server, both running
on the same machine, the address 127.0.0.1 can be used. This address
loops back to 127.0.0.0, the localhost.If you've got your own private network at home or at
work, then it's likely you're using
addresses such as 192.168.1.1 and 192.168.1.2. Addresses in the range
192.168.0.0 to 192.168.255.255 are reserved for this purpose and are
never used on the Internet. (There are also other ranges that are not
used on the Internet, but we don't discuss this in
detail here.)It's inconvenient to remember IP addresses. For
example, it's much easier for Hugh to remember that
hugh.cs.rmit.edu.au is his
machine at work than to remember its IP address is 131.170.27.120.
For this reason, most IP addresses have one or more equivalent domain
names that we use to log in, access web sites, and so on. The mapping
of IP addresses to names and back again is managed by domain name
servers. When you set up a new domain and host it on a server, the
domain name servers responsible for finding the system are usually
informed about the mapping between the machine's IP
address and your new domain name.
D.1.2.1 Ports
When a
virtual connection is set up between two communicating systems, each
end is tied to a port. The port is an identifier
used by the TCP software rather than an actual physical device, and
it allows multiple network connections to be made on one machine by
different applications.When a message is received by the TCP software running on a host
computer, the data is sent to the correct application based on the
port number. By convention, a well-known port is
normally used by a server providing a service that has seen
widespread adoption. A list of well-known ports for various
applications is maintained by Internet Assigned Number Authority
(IANA) and can be found at http://www.iana.org/assignments/port-numbers.
For example, the File Transfer Protocol (FTP) uses port 21, and a web
server uses port 80.Systems with TCP/IP software installed have a services file that
lists the ports used on that machine. This file is often
preconfigured for common applications and is maintained by the system
administrator to reflect the actual port usage on the machine. For
example, this file is usually /etc/services on a Unix system.
