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Acorn Omniclient User Guide


4 Introduction to TCP/IP

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.

You can use Internet and OmniClient NFS functionality to connect RISC OS computers directly to an existing Ethernet-based TCP/IP network. To do so, your RISC OS computers will need to have an Acorn Ethernet interface fitted.

Finding out more

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.

For details of how to use the TCP/IP Protocol Suite (Release 2), see the TCP/IP Protocol Suite (Release 2) User Guide.

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.

You should also see any relevant documentation supplied with other computers you plan to be on your TCP/IP network.

Finally, you can get more detailed information from Internetworking with TCP/IP. Douglas Comer (1988) Prentice-Hall, Englewood Cliffs, NJ, USA.

TCP/IP concepts

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.

If you've already got a TCP/IP network running…

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.

Host names

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.

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).

Interface names

Each network interface in each computer - whether it be an Ethernet or Econet 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.

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 saturn may have interfaces named saturn_eco and saturn_ether .

Internet addresses, netmasks and subnets

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.

An Internet address is four bytes long. These four bytes are split into fields:

network address
subnet address (optional)
host address

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.

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 decimal.

Unlike the interface name, the Internet address must be unique on all networks with which the interface will ever communicate.

If you plan to connect to other sites…

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:

DDN Network Information Center
SRI International
Room EJ217
333 Ravenswood Avenue
Menlo Park, CA94025


Depending on the size of your network, you will be allocated a Class 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:

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).

Note that separate Econets (i.e. those not connected together by Econet bridges) form separate subnets.

If you don't plan to connect to other sites...

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:

Figure 4.2 Suggested local TCP/IP numbering scheme

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:

EthernetMeaning EconetMeaning 1 on Ethernet 1 on Econet 2 on Ethernet 2 on Econet 3 on Ethernet so on up to... 255 on Ethernet 255 on Econet 256 on Ethernet 256 on Econet 257 on Ethernet so on...

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.

Physical Addresses

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

Further Information

If you require any further information, please contact Acorn Customer Services' ftp site to obtain Application Note 283.

This edition Copyright © 3QD Developments Ltd 2015
Last Edit: Tue,03 Nov 2015