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System Administration Guide: IP Services

Setting Up an IP Address Scheme

The number of machines that you expect to support affects how you configure your network. Your organization might require a small network of several dozen standalone machines that are located on one floor of a single building. Alternatively, you might need to set up a network with more than 1000 hosts in several buildings. This arrangement can require you to further divide your network into subdivisions that are called subnets. The size of your prospective network affects the following factors:

Administering Network Numbers

If your organization has been assigned more than one network number, or uses subnets, appoint a centralized authority within your organization to assign network numbers. That authority should maintain control of a pool of assigned network numbers, and assign network, subnet, and host numbers as required. To prevent problems, ensure that duplicate or random network numbers do not exist in your organization. If you are planning to transition to IPv6, see Chapter 17, Transitioning From IPv4 to IPv6 (Reference).

Designing Your IPv4 Addressing Scheme

After you receive your network number, you can then plan how to assign the host parts of the IPv4 address.

The following table shows the division of the IPv4 address space into network and host address spaces. For each class, “Range” specifies the range of decimal values for the first byte of the network number. “Network Address” indicates the number of bytes of the IPv4 address that are dedicated to the network part of the address. Each byte is represented by xxx. “Host Address” indicates the number of bytes that are dedicated to the host part of the address. For example, in a class A network address, the first byte is dedicated to the network, and the last three bytes are dedicated to the host. The opposite designation is true for a class C network.

Table 3–1 Division of IPv4 Address Space

Class 

Range 

Network Address  

Host Address 

A

0–127  

xxx

xxx.xxx.xxx

B

128–191  

xxx.xxx

xxx.xxx

C

192–223  

xxx.xxx.xxx

xxx

The numbers in the first byte of the IPv4 address define whether the network is class A, B, or C. InterNIC assigns the numbers. The remaining three bytes have a range from 0–255. The numbers 0 and 255 are reserved. You can assign the numbers 1–254 to each byte, depending on the network number that is assigned to you.

The following table shows which bytes of the IPv4 address are assigned to you. The following table also shows the range of numbers within each byte that are available for you to assign to your hosts.

Table 3–2 Range of Available Numbers

Network Class 

Byte 1 Range 

Byte 2 Range 

Byte 3 Range  

Byte 4 Range 

A

0–127 

1–254 

1–254  

1–254 

B

128–191 

Preassigned by Internet 

1–254 

1–254 

C

192–223 

Preassigned by Internet 

Preassigned by Internet 

1–254 

How IP Addresses Apply to Network Interfaces

In order to connect to the network, a computer must have at least one network interface. Each network interface must have its own unique IP address. The IP address that you give to a host is assigned to its network interface, sometimes referred to as the primary network interface. If you add a second network interface to a machine, the machine must have its own unique IP number. When you add a second network interface, the machine changes to a router. See Configuring Routers for an explanation. If you add a second network interface to a host and you disable routing, the host is then considered a multihomed host.

Each network interface has a device name, device driver, and an associated device file in the /devices directory. The network interface might have a device name, such as le0 or smc0, device names for two commonly used Ethernet interfaces.

Note –

This book assumes that your machines have Ethernet network interfaces. If you plan to use different network media, refer to the manuals that come with the network interface for configuration information.