Chapter 2. Planning a Network

This chapter contains common-sense approaches to planning the physical and logical aspects of your network environment. The information contained in this chapter should be read before you set up a new network or integrate into an existing network.

This chapter contains the following sections:

Planning the Physical Network

Planning the physical network requires that you first answer the question, “What network media and topology configuration would best suit the needs of my users?” A review of the MAC (Medium Access Control) level and application-level performance information about the products you are considering will help you determine the appropriate choice of media for your environment. In your review, consider the size (number of stations) of your network. Your network size will influence the media type and topology you choose for your network. If your network requires different types of media, determine whether you have the correct equipment for integrating the various media types.

These subsections will help you answer this list of planning questions:

About Repeaters, Bridges, Routers, and Gateways

Your choice of media and the number of stations, networks, and protocols in your network may require the use of a repeater, bridge, router, or gateway. This section suggests the type of device required for certain network functions.


A device that regenerates and amplifies electrical signals. Its purpose is to extend the physical length of a network.


A device that decodes MAC-layer frames transmitted between different hardware and media. Its purpose is to resolve network media differences; it allows a network to be composed of various media types (Ethernet, fiber, serial, and so on). It can also be used to segment similar media types and provide segment isolation for lower network traffic .


A device that decodes and passes network-layer packets between different networks. Its purpose is to provide the physical and logical route from one network to another.


A device that translates protocols from one station to another. Its purpose is to allow stations with different networking protocols to communicate successfully.

Note: The terms router and gateway are sometimes used interchangeably. Be sure you know the function of the device you are considering, because the term may be technically inaccurate.

Note that each device may not be limited to a single function. For example, a gateway may also perform router functions if it is configured as a router. Table 2-1 summarizes the characteristics of each network device.

Table 2-1. Network Device Characteristics

Device Name









extends physical length of the network





bridge network media differences





provides physical and logical route between networks





communication between stations with different networking protocols

About Network Performance

You can circumvent some performance bottlenecks with appropriate planning. These bottlenecks might occur as a result of your choice of media, topology, number of network devices, controller boards, or network design.

Choice of media

Be sure the capacity of the medium you have selected is adequate for the network size and data transmission type (large or small volumes of data, sporadic or steady traffic). For example, Ethernet has a range of capacities depending on the specific type of Ethernet cable used (10base5, 10base2, 10baseTor 100baseT). Media type is also a factor in data degradation. For example, 10baseT is category 3 unshielded twisted pair and is more sensitive to environmental conditions than 10base5. 100baseT must be category 5 unshielded twisted pair. These are considerations if you are planning a network for a manufacturing environment that produces a high degree of electrostatic discharge.

Number of devices

Network devices can cause degradation to the network performance. Use repeaters only when necessary to amplify the signal. Each additional device introduces additional resistance onto the network.

Choice of controller

Choose the most efficient controller for your media. For example, Silicon Graphics supplies a standard Ethernet controller. An optional Efast card handles more of the protocol processing in hardware and frees the station's CPU for other processing.

Design of network

Think about the design of your network before you begin setting it up. If possible, put departments that interact heavily on the same network to decrease router traffic. Use dedicated routers to handle heavy traffic between networks.  

About Wide Area Networks

In addition to the many options available for constructing local area networks, there are several different ways of connecting local area networks into wide area networks. These systems can be used to tie together local area networks at different locations, to allow users working at scattered locations to access a network, and to connect your network to the outside world. These subsections introduce the different systems:

Figure 2-1 shows how different kinds of wide-area connections might fit into a large heterogeneous network.

Two of the available systems, Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP), provide a way of transferring Internet Protocol (IP) packets over a serial telephone line; this means that SLIP and PPP users can access network resources much as if they were on the local area network. PPP can also be used with Integrated Services Digital Network (ISDN), available on certain platforms. ISDN uses a high-speed digital telephone line to achieve higher throughput than is possible with a modem connection.

Another system, UNIX to UNIX Copy Program (UUCP), is an older system, primarily designed for transferring information (such as network news and electronic mail) in batch mode over serial lines.

Higher-performance network connections can be made using specialized hardware. These connections are usually over dedicated lines, leased from a telephone company, or over the telephone company's packet-switched network.

Figure 2-1. Heterogeneous Network With Wide-Area Connections

Heterogeneous Network With Wide-Area Connections

Serial Line Internet Protocol (SLIP)

SLIP provides simultaneous operation of multiple processes on a serial cable or telephone line. It allows network users the freedom to use TCP/IP based applications over a serial cable or modem connection.

You might consider setting up a SLIP network when cost and distance are large factors in your network planning.

Point to Point Protocol (PPP)

The Point to Point Protocol is similar in nature to SLIP. PPP provides a network connection as if your system were connected to the remote host by a LAN connection. Multiple processes and TCP/IP based applications are supported.

UNIX to UNIX Copy Program (UUCP)

UUCP, also called the Basic Networking Utilities (BNU), is a set of utilities that lets stations using a version of the UNIX operating system (such as IRIX) communicate with each other over serial lines. The utilities provided range from those used to copy files between computers to those used for remote login and command execution.

You may consider setting up UUCP for long-haul communications using modems and telephone lines. It is usually used to distribute electronic mail and network news.

Integrated Services Digital Network (ISDN)

ISDN is a system that connects systems using high-speed digital telephone lines. ISDN can achieve throughput up to 128 kilobits (Kb) per second, several times faster than normal modem connections. However, ISDN service can be expensive, and is not available in all areas, nor all platforms. See the ISDN User's Guide for more information on ISDN.

Internet Gateway

The Internet Gateway provides a server process to connect with the Internet and a means of configuring various name services. It acts as a router, and has its own system of help screens to help the user through the configuration process.

High Performance Wide Area Networks

If you need higher performance than you can get using SLIP or PPP over a modem link or ISDN, there are several choices available to you. The choices include Frame Relay networking and leased line service, from 56 Kb (56 Kb per second) to T1 (1.5 Mb per second) and T3 (up to 45 Mb per second). If you require this kind of service, you will have to shop around, comparing the prices and services offered by local Internet service providers.

Internet Protocol Addresses

Each system on your network needs a unique Internet Protocol (IP) address for each of its network interfaces. The Internet Network Information Center (InterNIC) is responsible for assigning the network portion of an Internet address for each site. For example, if Company A applies for an Internet address, the InterNIC provides the network portion of the Internet address for the entire Company A site. A centralized organization within Company A is responsible for assigning and managing the station ID portion of the Internet address.

Internet addresses are maintained on each station or in a centralized network database such as NIS or BIND. See “About Name-to-Address Mapping”. for a comparison of the different database types. Each station that wishes to communicate must have a valid Internet address registered in the appropriate database. The standard hosts name-address database on IRIX stations is the /etc/hosts file.  

These subsections will help you answer this list of planning questions:

Format of Internet Protocol (IP) Version 4 Addresses

An IP address is a 32-bit number that network software uses to identify a system on a network. For the sake of human readability, these addresses are usually represented as four one-byte integers, separated by dots (for example, Every system on an IP network must have its own unique IP address for the network to function properly. Systems with more than one network interface must have a unique IP address for each interface.

Note: Unlike a system's Ethernet address, a system's IP address is determined by the network and network system administrators.

Conceptually, each 32-bit IP address is a pair of numbers where one number represents the network and the other the system itself. There are four classes of addresses in use (A through D). The class of address is determined by the first bits of the address:

  • Class A addresses begin with 0 and have 7 bits for the network number and 24 bits for the host number.

  • Class B addresses begin with 10 and have 14 bits for the network number and 16 bits for the host number.

  • Class C addresses begin with 110 and have 21 bits for the network number and 8 bits for the host number.

  • Class D addresses begin with 1110 and are special “multicast” addresses for use within a network site.

In all cases, host numbers 0 and 255 are reserved, and may not be used for actual systems.

Figure 2-2 shows the format of the different classes of Internet addresses.

Figure 2-2. Format of Internet Protocol (IP) Version 4 Addresses

Format of Internet Protocol (IP) Version 4 Addresses

To simplify Internet addressing, dotted decimal notation is used to break the 32-bit number into four decimal numbers separated by dots.

For example, the IP address in binary is

10000000| 01001010| 00101001| 01111011


128 | 74 | 41 | 123

Class A, B, and C IP addresses in dot notation conform to the following specifications:

Class A -- 001.hhh.hhh.hhh through 126.hhh.hhh.hhh
Class B -- 128.001.hhh.hhh through 191.254.hhh.hhh
Class C -- 192.000.001.hhh through 223.255.254.hhh

Note: hhh is the local system and the leading numbers are the network.

Networks are usually identified by network numbers—IP addresses in which the host portion is not specified. For example, 150.166 represents a Class B network, and 192.26.80 represents a Class C network.

If your network will be connected to the Internet, then you must obtain a unique network number, as described in “Obtaining a Network Number”.. All the systems on your network must have IP addresses allocated from your network.

If you are adding a machine to an existing network, its IP address must be allocated from that network.

Obtaining a Network Number

You should obtain an Internet network number before you begin setting up your network.The allocation of network numbers is managed by a set of organizations called Network Information Centers (NICs). (See “About Local Network Information Centers”.) If your network is going to be isolated, and will never be attached to the Internet, you can theoretically use any addresses you like. However, if your network is ever going to be attached to the Internet, you should obtain a valid network number. Before you request the network number, you should determine the current needs of your organization (how many systems do you currently have that should be on the network?) and expected growth over the next five years.

There are several ways to obtain a network number. In many cases the best option, if you are connecting to the Internet through an Internet service provider, is to have the service provider assign you a portion of the address space they have been allocated by the local NIC.

The InterNIC recommends that you request a network number from your network service provider. If they cannot supply one, contact your provider's provider. As a last resort, contact your Network Information Center. See “About Local Network Information Centers”.

Required Information for Obtaining an Internet Address

To request an Internet network address, you typically need to supply the following information to the local NIC:

  • Your administrative point of contact (POC). The administrative POC is the person responsible for answering administrative and policy questions about the network. You need to know his/her name, title, mailing address, and phone number.

  • Your technical point of contact (POC). The technical POC is responsible for the technical support of the network. You need to know his/her name, title, mailing address, and phone number.

  • Organization name and postal address.

  • Your network name (up to 12 characters).

  • Your network's geographic location and organization name.

  • The name and location of the network document plan.

  • Gateway information (connectivity, hardware, software, address).

  • The approximate size of your network (number of hosts and subnets), initially and within one year.

  • Type of network (research, educational, government non-defense, commercial).

If you already have one or more network numbers assigned to your organization, the NIC may require you to provide information on how these are being used, as evidence that you really need a new network number.

If you request 16 or more Class C network numbers, the InterNIC requires you to provide information on network topology, and if you request 256 or more Class C network numbers or a Class B network number, the InterNIC requires you to provide a diagram of the proposed network.

Internet Protocol Version 6

This section describes Internet Protocol Version 6 (IPv6) and covers the following topics:

IPv6 Overview

Internet Protocol Version 6 (IPv6) is the next generation Internet protocol designed to eventually replace the current IPv4 (see “Internet Protocol Addresses” and “Format of Internet Protocol (IP) Version 4 Addresses”).

IPv4 uses a 32-bit address scheme to represent a unique Internet address while IPv6 uses a 128-bit addressing scheme. Because IPv6 has four times the number of bits to represent addresses, it provides many billion times the address space of IPv4 and adds an almost unlimited number of available IP addresses. This resolves a projected future shortage of IPv4 addresses as more and more devices are added to the Internet.

The IPv6 address scheme provides stateless, automatic configuration; improved service capabilities for services such as real-time; and enhanced support for authentication and privacy capabilities. It can help to restore end-to-end architecture in networks. To ease the transition from IPv4-based networks to IPv6-based networks, IRIX supports a dual-stack interface that supports both IPv4 and IPv6 protocols on the same system. The IPv6 basic socket API supports applications that transparently use the correct Internet Protocol. Applications can choose the right protocol to communicate with any particular node.

IPv6 has an encapsulation mechanism called tunneling that allows IPv6 packets to operate over IPv4 networks. The IPv6 protocol allows networking applications to operate transparently in both environments.

IRIX supports IPv6 using Berkeley sockets system calls. The differences between using sockets for IPv4 networking and IPv6 networking are minimal. The UDP and TCP transport layer protocols work in exactly the same way over IPv6 as they do over IPv4. For more information on IPv6 socket creation and socket options, see the ipv6(7P) man page.

The IRIX Network Programming Guide also includes information about IPv6 socket creation and socket options.

For a list of Internet Engineering Task Force (IETF) request for comments (RFCs) documents that describe the IPv6 functionality supported by IRIX, see the ipv6(7P) man page.

IPv6 Addressing

IPv6 supports unicast and multicast addressing. Descriptions are as follows:

  • unicast

    An IPv6 address for a single interface.

  • multicast

    An identifier for a set of interfaces that typically belong to different nodes.

IPv6 has different types of unicast addresses. A link-local unicast address is used over a single link. A global unicast address is used across a global Internet.

The ndpd(1M) daemon is invoked at boot time to perform IPv6 neighbor discovery for the automatic configuration of link-local addresses and to automatically configure global addresses when router advertisements are received from an IPv6 aware router.

The neighbor discovery protocol (NDP) is used to dynamically map between IPv6 Internet and device addresses such as Ethernet addresses. It is used by all the Ethernet interface drivers, although it is not specific to Ethernet.

The ndpd daemon is normally started by the network start-up script during boot and is run with the options contained in /etc/config/ndpd.options. When it starts, it first starts the NDP on each interface by choosing a link-local address and sending a neighbor solicitation message to check that the address is unique for that interface. It also sends out router solicitation messages on each interface. When a router advertisement message is received, it will try and configure a global address on that interface using the prefix contained in the message.

The route6d daemon is a routing daemon that supports routing information protocol (RIP) requests over IPv6.

Additional information on IPv6 topics described in this overview are available as follows:

  • For more information on the ndpd daemon, see the ndpd(1M) man page.

  • For more information on neighbor discovery protocol (NDP), see the ndp(7P) man page.

  • For more information on the route6d daemon, see the route6d(1M) man page.

  • For detailed information on NDP, see RFC 2461, Neighbor Discovery for IP Version 6 (IPv6).

  • For detailed information on IPv6 addressing, see RFC 2373, IPv6 Addressing Architecture.

Format of Internet Protocol IP Version 6 Addresses

IPv6 uses a 128-bit number to represent an IP address as shown in Figure 2-3.

Figure 2-3. Format of Internet Protocol (IP) Version 6 Addresses

Format of Internet Protocol (IP) Version 6 Addresses

IPv6 addresses are classless.

IPv6 addresses can be represented as text strings as follows:

  • x:x:x:x:x:x:x:x

    Each x represents a hexadecimal value of a 16-bit piece of the address (see Figure 2-3).

    An example address follows:


    It is not necessary to write the leading zero in an individual field, but there must be at least one numeral in every field except when zeros are compressed, as in the next example.

  • x::x:x:x:x

    You can use the single double colon :: syntax to indicate multiple groups of 16-bits of zeros. The :: syntax can appear only once in an address. You can also use this double colon syntax to compress the leading or trailing zeros in an address.

    An example follows:

    1080::8:800:200C:417A represents the same address as 1080:0:0:8:800:200C:417A

  • x:x:x:x:x:x:d.d.d.d

    Each x represents the hexadecimal values of the six, high-order, 16-bit pieces of the address. Each d represents the decimal values of the four, low-order, 8-bit pieces of the address (standard IPv4 representation).

    An example follows:


In IPv6, the left, or most significant bits, of the IP number identify the network on which a machine resides and are the prefix bits. The right, or least significant bits, identify a machine residing on a network or subnetwork and are the interface identifier bits. The prefix and interface identifier bits are shown in Figure 2-4.

Figure 2-4. IPv6 Prefix Bits and Interface Identifier Bits

IPv6 Prefix Bits and Interface Identifier Bits

IPv6 Header Format

Figure 2-5 shows the IPv6 header format specified by IPv6 protocol. The IPv6 headers are all the same length and contain fewer fields than IPv4. IPv6 headers are terminated on 64-bit boundaries rather than 32-bit boundaries as in IPv4.

Figure 2-5. IPv6 Header Format and Data Portion of a Datagram

IPv6 Header Format and Data Portion of a Datagram

The IPv6 header fields are briefly described as follows:

  • In IPv6, the version field must equal 6.

  • The semantics of traffic class and flow label are not yet agreed upon and these fields have default values of zero.

  • Payload length defines the length of the packet payload.

  • Next header refers to a higher-layer protocol, such as TCP, or may indicate an IPv6 extension header.

  • Hop limit contains a value that is decremented as nodes forward the packet.

  • Source IP address is the 128-bit address of the originating node.

  • Destination IP address is the 128-bit address of the target recipient for the packet.

IPv6 supports optional, Internet-layer information that is encoded in separate headers called extension headers. Extension headers may be placed between the IPv6 header and the upper- layer header in a packet. There are a small number of such extension headers, each identified by a distinct next header value. For more information on extension headers, see RFC 1883 and RFC 2460.

Installing and Configuring IPv6

To enable IPv6 protocol on a system, perform the following:

  1. Use the inst(1M) utility to install the eoe.sw.ipv6 from your IRIX distribution media. The and subsystems are optional documentation subsystems.

  2. Enable IPv6 protocol within the kernel by using the systune(1M) command to set ip6_enable as follows:

    systune ip6_enable 1

If your system is a router, you need to enter the following command:

systune ip6forwarding 1

If your system is an IPv6 router, you also need to modify the arguments to ndpd and route6d. For information on how to do this, see the ndpd(1M) and route6d(1M) man pages.

If you want to process routing headers, enter the following command:

systune ip6forwsrcrt 1

Domain Names

If you are planning on putting your site on the Internet or exchanging e-mail with sites on the Internet, you should register a domain name with your local NIC. A domain name uniquely identifies your organization. For example, Silicon Graphics has the domain name

These subsections explain domain names and subdomains:

The Internet uses Domain Name Service (DNS) to map domain names to IP addresses. Therefore, even if you do not use DNS internally, you must provide DNS name servers on the Internet in order to connect your network to the Internet. You should have at least two name servers, a primary and a secondary server. For robustness, the secondary server should not be connected to the Internet through the same gateway as the primary server. Since many organizations are not big enough to have multiple gateways to the Internet, a common solution is to make a reciprocal arrangement with another organization to provide secondary name service for each other.

If you are connecting to the Internet through an Internet service provider, they may be able to provide name service for your organization, or help you locate someone to provide secondary name service if you are able to provide a primary name server.

Obtaining a Domain Name

As with network numbers, the registration of domain names is administered by the Network Information Centers. In some cases, there is a fee associated with holding a domain name. For example, the InterNIC currently charges a fee of $100 for the first two years, and $50 a year thereafter for domains under its jurisdiction.

You can register your domain name through your local NIC. See “About Local Network Information Centers” for contact information. When you register a domain, you should also register a reverse domain, also known as an IN-ADDR domain. The reverse domain provides a mapping from IP addresses to domain names.

In many cases, Internet service providers will register your domain for you, for a fee.

About Subdomains

Once you have a domain name registered, you are free to establish subdomains of your own. This is particularly useful for large organizations that use the Domain Name Service (DNS). The use of subdomains with DNS allows some administrative chores to be decentralized.

For example, suppose has branch offices in Gilroy and Paris. These could be established as subdomains,, and

About Internet Connections

Chances are, you will want to connect your system or network to the Internet. Wherever you may be, there is likely an Internet gateway available within your local calling range. The following sections offer some information that should help you get set up and running. Obviously, each situation is somewhat different, and your local service provider will have variations in service and equipment. Some research and experimentation is usually required before everything works smoothly.

Before Connecting to the Internet

Before you sign up for an internet connection, consider what level of service you need. For example, if you are an individual looking for basic e-mail, news, and file transfer capabilities, it probably would not make sense to install a dedicated network cable in your home for economic reasons. A better choice for single-user access might be to subscribe to a network provider who establishes an account for you on their system (one that is currently connected to the Internet). Typically, access to their system is through a modem connection.

If you are trying to establish a connection to the Internet for a corporation, you will likely need the bandwidth of a leased line, and all the required hardware that goes with it. You will have to take into consideration the many administrative issues of running a site. These issues include, but are not limited to

  • Cost analyses/budget

  • Establishing a domain

  • Applying for IP addresses

  • Establishing site policy

  • Establishing site security

  • Administration of network services (such as Domain Name Services, NIS, e-mail, and so on)

There are providers of network connectivity that can provide varying levels of service. You must investigate the providers, and decide who provides the level of service you need, at the appropriate cost.

If you choose an individual account on a provider's machine, the service provider deals with most, if not all, of the administrative tasks, and you simply enjoy access to the Internet.

If you would like a broader range of services, most providers will set you up with a dedicated modem and phone line for your exclusive use, or they can provide a network-only service (using SLIP, PPP, or UUCP), either through modems or other network connections.

Connecting your network to the Internet requires a number of steps, including arranging name servers, obtaining a network number, and registering a domain name for your organization. Many Internet service providers are willing to provide these services for a fee.

If you are trying to set up internet access for a company, or corporation, you should research the issues listed above. Based on the information you obtain, formulate a plan for your site based on the needs and expectations of your organization. One of the best sources of information is the Internet itself. You should first obtain an individual account from a local provider. With the individual account, you can gain access to a large amount of information pertaining to establishing a site on the Internet.

About Local Network Information Centers

Before you connect your site to the Internet, you will need to contact your local Network Information Center. The assignment of network numbers and domain names is coordinated by the Network Information Centers. There are three main regional Network Information Centers, as shown in Table 2-2.

Table 2-2. Network Information Centers




Asia Pacific Network Information Center (APNIC)


Réseaux IP Européens Network Coordination Centre (RIPE NCC)


Internet Network Information Center (InterNIC)

other areas


Procedures for obtaining IP addresses and registering domain names vary, so contact your local NIC for information.

Internet Network Information Center

The Internet Network Information Center (InterNIC) was formerly the sole Network Information Center. It serves as the primary NIC for most of North and South America, as well as for other regions that do not yet have NICs of their own. InterNIC maintains a large archive of informational documents, which can be accessed using WWW, FTP, or by e-mail to an automated-response mail server. Registration authority for some countries (including Canada and Brazil) is delegated to national NICs. Contact information for the national NICs may be obtained through InterNIC.

Network Solutions
Attn: InterNIC Registration Services
505 Huntmar Park Drive
Herndon, VA 22070
Phone: 1-800-444-4345 or 1-703-742-4777
E-mail: [email protected] (general inquiries)
E-mail: [email protected] (registration services)
FTP: (complete RFCs, and so on)
FTP: (registration information)
E-mail server: [email protected] (send message with subject line “HELP”)

Réseaux IP Européens

Réseaux IP Européens (RIPE) maintains an NIC that provides registration services for European sites. It also maintains a store of informational documents, including the InterNIC's FYI documents, and instructions on how to register a host or network in one of the European domains.

RIPE Network Coordination Centre
Kruislaan 409
NL-1098 SJ Amsterdam
The Netherlands
Phone: +31 20 592 5065
Fax: +31 20 592 5090
E-mail: [email protected]

Asia Pacific Network Information Center

The Asia Pacific Network Information Center (APNIC) coordinates network information for the Asia and Pacific region. Registration authority for some countries is delegated to national NICs. Contact information for the national NICs may be obtained through APNIC.

Asia Pacific Network Information Center
c/o United Nations University
53-70 Jingumae 5-chome
Shibuya-ku, Tokyo 150
Phone: +81-3-5467-7014
Fax: +81-3-5276-6239
E-mail: [email protected]

Online Information Sources

With an individual account or other access to the Internet, you can get the information you need to provide access to your own site.

Usually, the provider of an individual account will also provide new-user documentation that describes the basics of using the Internet. You can use the World Wide Web (WWW) and the File Transfer Protocol (FTP) to access a wealth of information on many subjects, including Internet connectivity. If you do not know how to use FTP, see “Retrieving Files With Anonymous FTP” for a short tutorial. How you access the Web depends on what Web browser you are using. Most Web browsers have online help available.

The following subsections will help you use the Internet:

Network Information Centers

Your local NIC maintains archives of useful information on connecting to the Internet. In addition to information about requesting network numbers and registering domain names, they may have lists of local service providers. Most NICs make this information available by WWW and FTP. See “About Local Network Information Centers” for WWW and FTP addresses for the major NICs.

The InterNIC has produced a series of information bulletins called FYIs. Especially notable is FYI 16, entitled Connecting to the Internet-—What Connecting Institutions Should Anticipate. While this is aimed primarily at U.S. educational institutions, it remains one of the better pieces of documentation on establishing a site on the Internet. The FYI documents are available by WWW and FTP from the InterNIC and from RIPE.

Internet Society

The Internet Society is a non-governmental international organization for global cooperation and coordination of the Internet. They also provide useful online information—in particular, information on finding an Internet service provider, and a list of network service providers around the world. This information is available by WWW. A subset is available by anonymous FTP.


Retrieving Files With Anonymous FTP

Anonymous FTP is a conventional way of allowing you to sign onto a computer on the Internet in order to obtain copies of files that are made available to the public. Some sites offer anonymous FTP accounts to distribute software and various kinds of information. If you have never used ftp, here is a brief summary on how to use the ftp command. To connect to a remote host, specify the hostname on the command line:


When ftp connects with the remote system, it prompts you for a login name. Use the login name “anonymous”:

Connected to
Name ( anonymous
331 Guest login ok, send ident as password.

Many systems allow any password and request that the password you choose is your user ID. If this fails, the generic password is usually “guest.”

230 Guest login ok, access restrictions apply.
Remote system type is UNIX.
Using binary mode to transfer files.

Once connected and logged in, you can use ftp's cd and ls commands to look at the files available on the remote system. To obtain a file from the remote system, use the get command. The get command copies one file from the remote system to your local system. To obtain multiple files from the remote system, use the mget command.

ftp> cd fyi
250 CWD command successful.
ftp> get fyi6.txt
local: fyi6.txt remote: fyi6.txt
200 PORT command successful.
150 Opening BINARY mode data connection for fyi6.txt (3459 bytes).
226 Transfer complete.
3459 bytes received in 0.46 seconds (7.34 Kbytes/s)

About Name-to-Address Mapping

Because IP addresses are difficult to remember, they are usually associated with names. In the case of a machine with a single IP address, this name usually consists of the machine's hostname and domain name. For example, a machine called fruit in the domain would usually be referred to as For clarity, this type of name will be referred to in this section as a network connection name.

Because network connection names usually correspond to the machine's hostname, these network connection names are commonly referred to as “hostnames,” but this can be misleading. The actual hostname is defined in the /etc/sys_id file. By default, this hostname is used as the network connection name for the machine's primary network interface, but this behavior is configurable. A machine with multiple network interfaces has multiple network connection names associated with it. By convention, each of these connection names contains the hostname—for example, if the host fruit acts as a gateway between two networks in the domain, it might use these names:

The process of mapping network connection names to IP addresses is commonly called hostname resolution. There are several different systems for hostname resolution. Machines can use a local database (the /etc/hosts database), or they can obtain information from servers on the network, using either the Network Information Service (NIS) or the Domain Name System (DNS). The following sections describe the advantages and drawbacks of the different systems:

/etc/hosts Database

The /etc/hosts database is an ASCII file that you can modify with any text editor. The file contains lines of text that specify IP addresses and network connection names.

For a small network of stations under the same administrative control, maintaining a consistent /etc/hosts database is straightforward. Establish a master copy on one station and make additions or deletions from its file. Then use rcp or rdist to copy the file to the other stations in the network.

Maintaining consistent versions of /etc/hosts on every station in a large network is troublesome. NIS and the BIND name server both make maintenance easier by providing a centralized version of the host database.

Domain Name System

The Internet uses the Domain Name System (DNS) to map names to IP addresses. The most common implementation of a DNS name server is called Berkeley Internet Name Domain (BIND). If your network interfaces with the Internet, you must have at least two DNS name servers, a primary and a secondary server. Your Internet service provider may be able to take care of this requirement for you.

BIND is best suited for large networks, or networks connected directly or indirectly to the Internet. BIND provides access to a much larger set of stations than is provided in the /etc/hosts database. A drawback of BIND is its complicated setup. BIND is described in more detail in Chapter 6, “BIND Name Server”.

Network Information Service (NIS)

NIS is a network-based information service and an administrative tool. It allows centralized database administration and a distributed lookup service. NIS supports multiple databases based on regular text files. For example, NIS databases can be generated from the hosts, passwd, group, and aliases files on the NIS master.  

NIS is best suited for a moderate-sized network (one containing approximately 1000 stations, or a small collection of interconnected networks). NIS is part of the NFS optional software and is detailed in the NIS Administration Guide.

Guidelines for a Subnetwork

Subnetting allows you to divide a single network into a set of subnetworks. Subnetworks are useful for many reasons. For example, if you have a satellite office that connects to your main network, it should have its own network number or subnet. If you have a large number of systems to be connected by Ethernet, you may have to use subnets to overcome physical limitations on the number of hosts and length of network cable that can be supported on a single Ethernet network.  

Subnetting should be considered when the class limits are unrealistic for your network. For example, a Class B network gives you approximately 64,000 stations per network. This far exceeds the maximum number of stations allowed on most networks. Subnetting allows the local organization to designate some of the host ID bits to form a subnet. Subnetting generates a realistic number of stations per network. All changes are made at the local site by the site administration group and are transparent to off-site stations.

Planning is required for subnetting a network (see “Subnetting a Network” in Chapter 3 for subnetting procedure). Primarily, you must determine how to partition the host part of the 32-bit Internet address. To define local subnetworks, use bits from the host number sequence to extend the network portion of the Internet address. This reinterpretation of IP addresses is done only for local networks. It is not visible to off-site stations. You should have at least a rough idea of the physical layout of the network before you plan your subnets. For example, you might want to have a subnet for each floor of your building. If you have a branch office that's connected to your main network, you might want to set aside one or more subnets for it. In some cases, you may want to set aside a subnet for SLIP and PPP clients (see “SLIP and PPP Routing and Address Allocation” in Chapter 5).

Sites with a Class A network number have 24 bits of host part with which to work; sites with a Class B network number, 16 bits; and sites with a Class C network number, 8 bits. For example, if your site has a Class B network number, each station on the network has an Internet address that contains 16 bits for the network number and 16 bits for the host number. To define 254 local subnetworks, each possessing at most 254 stations, you can use 8 bits from the host portion of the address. Construct new network numbers by concatenating the original 16-bit network number with the extra 8 bits containing the local subnetwork number.

Note: It is highly recommended that the size of the portion of the host number sequence used for a subnet id be the same for all subnets. Variable subnets are supported but difficult to get correct.

Figure 2-6 shows what happens to the bit assignments in a Class B Internet address that is subnetted.

Figure 2-6. Subnetted Class B Address

Subnetted Class B Address

For example, the Class B Internet address for an entire site as seen from other sites might be 128.50. If subnetting is enabled within the site, the site might be composed of several subnets with network IDs like 128.50.20, 128.50.21, 128.50.22, and so on. A station that resides on the subnet 128.50.21 might have the Internet address

Note: Numbers consisting of all 0s, and all 1s, are reserved for broadcast addresses. Do not use subnetwork numbers with all 0s or all 1s.

IP Address Allocation

Once you have obtained a network number, decided which system to use for hostname resolution, and decided whether you are going to subnet the network, you are ready to allocate IP addresses for individual systems. For most systems, this is as simple as assigning an unused IP address from the correct net or subnet. If you use the syntax of the /etc/hosts file, the result will look something like this: green tossed jello
<IP address>   <host>.<domain> <host> <alias>

Note: Host numbers 0 and 255 are reserved, and shouldn't be used.

Systems with more than one network interface may be connected to more than one subnet, and require one address for each connected interface. Each interface should be assigned an address from the subnet that the interface is connected to. For example, if acts as a gateway between the 150.26.80 net and the 150.26.42 net, it might have the following entries: fruit gate-fruit

Even if you are planning on using NIS or BIND for hostname resolution, you will probably want to put together an /etc/hosts file. If you install this on your systems as you attach them to the network, you will be able to communicate while you get NIS or BIND up and running.

You should also establish some policy for allocating IP addresses for new systems once the network is in place. If your organization is large, you might want to delegate this authority to separate organizational units. For example, the branch office with its own subnet should allocate IP addresses as needed from its subnet. If your organization is divided up into subdomains, you might want to assign authority over certain subnets to subdomain administrators.

About Network Security

Securing a network is difficult. If you can discourage potential intruders and quickly isolate or pinpoint successful intruders, you can consider your network secure. You should establish a plan for keeping your network secure before you connect your network to the Internet. For information on network security, see Chapter 5, “Network Security,” in IRIX Admin: Backup, Security, and Accounting .

Common Network Applications

This guide is written specifically to support the standard network hardware and software—Internet protocols over Ethernet. However, when discussing networking in general, it is difficult to ignore network applications that are not standard, but are common to most network environments.

These subsections present a brief overview of some of the common network applications that you should consider when planning your network:

About Electronic Mail

Electronic mail is a group of programs (sendmail) used to send and receive messages to and from users on the same local station or between remote stations. Mail can be sent using UUCP or TCP/IP protocols. IRIX supports both System V (/bin/mail) and 4.3BSD (/usr/sbin/Mail) mail programs, as well as most other mailers including Netscape Mail, which provides a graphical interface for electronic mail.

About Network File System (NFS)

NFS is a network program that can access a remote station's filesystem and attach it and its data to the local station's filesystem. On the local station, the remote filesystem is accessed as if it were local.  

NFS should be considered in a network when you want to share files between stations. With NFS, software or data used by a group is put on an NFS server. Authorized NFS clients access the data over the network when needed. This approach ensures consistent information, frees up disk space on client stations, and simplifies the backup procedure. NFS is an optional software product and is described in the ONC3/NFS Administrator's Guide.

Note: NFS is not included with the IRIX operating system and must be purchased separately.