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<TITLE>Introduction to TCP/IP</TITLE>
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<H1>4 Introduction to TCP/IP</H1>
<HR>
<P>
OmniClient enables you to connect Acorn RISC OS computers to a
TCP/IP network, and to access computers on that network in a variety
of ways.
<P>
You can use Internet and OmniClient NFS functionality to connect
RISC OS computers directly to an existing <A NAME="Ether">Ether</A>net-based
TCP/IP network. To do so, your RISC OS computers will need to
have an Acorn Ethernet interface fitted.
<H3>Finding out more</H3>
<P>
For general information on the use of a RISC OS computer and its
desktop interface, see the Welcome Guide and RISC OS 3 User Guide
supplied with it.
<P>
For details of how to use the TCP/IP Protocol Suite (Release 2),
see the TCP/IP Protocol Suite (Release 2) User Guide.
<P>
For details of how to use the programming interfaces provided
by the TCP/IP Protocol Suite (Release 2), see the TCP/IP Protocol
Suite (Release 2) Programmer's Guide, available separately from
Acorn Developer Support and on the Acorn ftp site (although unsupported).
This includes a disc of useful C libraries.
<P>
You should also see any relevant documentation supplied with other
computers you plan to be on your TCP/IP network.
<P>
Finally, you can get more detailed information from Internetworking
with TCP/IP. Douglas Comer (1988) Prentice-Hall, Englewood Cliffs,
NJ, USA.
<H2><A NAME="TCP">TCP</A>/IP concepts</H2>
<P>
When you install OmniClient, you will have to assign certain names
and numbers to the computers on your TCP/IP network, and to their
network interfaces. This section explains those names and numbers.
<H4>If you've already got a TCP/IP network running </H4>
<P>
If you've already got a TCP/IP network running on your site, you
should already have naming and numbering schemes set up. Make
sure that any names and numbers you assign conform to this scheme,
and that you first contact the person who administrates their
allocation.
<H3><A NAME="Host">Host</A> names</H3>
<P>
Each computer on your network must have a principal host name,
or host name for short. Your users will use this name to refer
to the computer. The name must be unique on your site - you can't
have two computers with the same name.
<P>
It helps your users if each host name is easy for them to remember.
One way to do this is to use a theme, such as planets (e.g. saturn,
uranus); another way is to give names that have some relationship
to the computer's function on your network (eg accounts1, accounts2).
You can combine these ideas - so you might name the graphics department's
computers after famous artists (eg turner, vangogh).
<H3>Interface names</H3>
<P>
Each network interface in each computer - whether it be an Ethernet
or <A NAME="Econet">Econet</A> interface - must also have an interface
name. Again, this name must be unique on your site - you can't
have two interfaces with the same name.
<P>
If there's only a single interface in a computer it's normal to
use just the principal host name as the interface name. If there
are two interfaces in a name it's normal to refer to the principal
host name in each interface name: so a machine named <TT><FONT FACE="Courier">saturn</FONT></TT>
may have interfaces named <TT><FONT FACE="Courier">saturn_eco
</FONT></TT> and <TT><FONT FACE="Courier">saturn_ether</FONT></TT> .
<H3>Internet <A NAME="addr">addr</A>esses, <A NAME="netmask">netmask</A>s
and subnets</H3>
<P>
Furthermore, each interface must also have a unique numerical
address, known as its Internet address. It is this address that
the TCP/IP protocol uses to communicate; if a user specifies a
host name or interface name, the software automatically converts
it to an Internet address.
<P>
An Internet address is four bytes long. These four bytes are split
into fields:<BR>
<P>
<TABLE BORDER=1>
<TR><TD ALIGN="CENTER" VALIGN="TOP" WIDTH=149><CENTER><I><B>network address</B></I></CENTER>
</TD><TD ALIGN="CENTER" VALIGN="TOP" WIDTH=208><CENTER><I><B>subnet address </B></I><B>(optional)</B></CENTER>
</TD><TD ALIGN="CENTER" VALIGN="TOP" WIDTH=234><CENTER><I><B>host address</B></I></CENTER>
</TD></TR>
</TABLE>
<P>
The network address identifies an entire network (which is typically
a whole site). The subnet address is optional, and identifies
a local network that forms part of the main network. The host
address identifies a host on that network.
<P>
A netmask specifies the portion of the address used by the network
and subnet addresses. For example, if the network address is held
in the top byte, and no subnets are used, the netmask would be
0xFF000000 (i.e. FF000000 hexadecimal) or 255.0.0.0 decimal.
<P>
Unlike the interface name, the Internet address must be unique
on all networks with which the interface will ever communicate.
<H4>If you plan to <A NAME="plan">connect to other sites </A>
</H4>
<P>
If you plan to connect to other sites over the Internet, you need
to ensure not only that Internet addresses are unique to your
site, but also that they are unique to the entire Internet. The
Internet already connects together thousands of sites, each with
many hosts. Clearly it's impossible to keep so many Internet addresses
unique on an informal basis. Consequently there is an administrative
body responsible for allocating network addresses. You must contact
them before you use the Internet to connect to other sites; write
or send email to:
<P>
<A NAME="DDN">DDN</A> Network Information Center<BR>
SRI International<BR>
Room EJ217<BR>
333 Ravenswood Avenue<BR>
Menlo Park, CA94025<BR>
USA
<P>
email: <TT><FONT FACE="Courier">HOSTMASTER@SRI-NIC.ARPA</FONT></TT>
<P>
Depending on the size of your network, you will be allocated a
<A NAME="Class">Class</A> A, B or C address: these use respectively
the top one, two or three bytes for the network address. It is
your responsibility how you use the remaining unallocated bytes
to specify subnets and hosts. For example, let's say you`ve been
allocated a Class B network address, and so have two bytes free
for your own use:
<UL>
<LI>If all your site's computers are connected to a single local
network, you won't need to use subnets, and so might use all two
bytes for the host address (allowing 64k hosts). In this case,
you'd use a netmask of 0xFFFF0000.
</UL>
<P>
Note that an Ethernet generally behaves as a single network, even
if it is made up of multiple segments of cable (unless divided
by routers).
<UL>
<LI>However, if your site's computers are connected to different
local networks (such an Ethernet and some Econets) you'll need
to use subnets. You might decide to use 5 bits for the subnet
address (allowing 32 subnets), and the remaining 11 bits for the
host address (allowing 2000 hosts). In this case, you'd therefore
use a netmask of 0xFFFFF800.
</UL>
<P>
Note that separate Econets (i.e. those not connected together
by Econet bridges) form separate subnets.
<H4>If you don't plan to connect to other sites...</H4>
<P>
If you don't plan to connect to other sites over the Internet,
all you need to do is to ensure that the interface's Internet
address is unique on your own site. We suggest you use the following
scheme:
<P>
Figure 4.2 Suggested local TCP/IP numbering scheme
<P>
Number your local networks from ten: for example, you might number
your Ethernet as net 10, and an Econet as net 20. Likewise, number
your hosts (not your interfaces) from 10. Your available Internet
address and their meanings would be as follows:
<P>
<TABLE BORDER="1">
<TR><TD WIDTH=95><B>Ethernet</B></TD><TD WIDTH=161><B>Meaning</B>
</TD><TD WIDTH=113><B>Econet</B></TD><TD WIDTH=170><B>Meaning</B>
</TD></TR>
<TR><TD WIDTH=95>10.0.0.1</TD><TD WIDTH=161>host 1 on Ethernet
</TD><TD WIDTH=113>20.0.0.1</TD><TD WIDTH=170>host 1 on Econet
</TD></TR>
<TR><TD WIDTH=95>10.0.0.2</TD><TD WIDTH=161>host 2 on Ethernet
</TD><TD WIDTH=113>20.0.0.2</TD><TD WIDTH=170>host 2 on Econet
</TD></TR>
<TR><TD WIDTH=95>10.0.0.3</TD><TD WIDTH=161>host 3 on Ethernet
</TD><TD WIDTH=113>20.0.0.3</TD><TD WIDTH=170>and so on up to...
</TD></TR>
<TR><TD WIDTH=95>10.0.0.255</TD><TD WIDTH=161>host 255 on Ethernet
</TD><TD WIDTH=113>20.0.0.255</TD><TD WIDTH=170>host 255 on Econet
</TD></TR>
<TR><TD WIDTH=95>10.0.1.0</TD><TD WIDTH=161>host 256 on Ethernet
</TD><TD WIDTH=113>20.0.1.0</TD><TD WIDTH=170>host 256 on Econet
</TD></TR>
<TR><TD WIDTH=95>10.0.1.1</TD><TD WIDTH=161>host 257 on Ethernet
</TD><TD WIDTH=113>20.0.1.1</TD><TD WIDTH=170>and so on...</TD>
</TR>
</TABLE>
<P>
Of course if a machine has only got one interface fitted, you'll
only use one of the addresses assigned to it; one of the addresses
will be wasted. But if you later upgrade the machine to add a
second interface, you already have a meaningful Internet address
reserved for it.
<H3>Physical Addresses</H3>
<P>
Each interface also has a six byte physical address (alternatively
known as its MAC address). You shouldn't need to do anything to
set this up, because
<UL>
<LI>an Ethernet interface's physical address is unique worldwide,
and is set in the hardware at the time of manufacture.
<LI>an Econet interface's physical address is based on its network
and station numbers. So long as you've correctly installed the
Econet, these should be unique to your site.
</UL>
<H2>Further Information</H2>
<P>
If you require any further information, please contact Acorn Customer
Services' ftp site to obtain Application Note 283.
<P>
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