System Administration Guide, Volume 3

Setting Up an IP Addressing Scheme

The number of machines you expect to support will affect several decisions you need to make at this stage of setting up a network for your site. Your organization might require a small network of several dozen standalone machines 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 called subnets. The size of your prospective network will affect:

Network class you apply for
Network number you receive
IP addressing scheme you use for your network

Obtaining a network number and then establishing an IP addressing scheme is one of the most important tasks of the planning phase of network administration.

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, assigning network, subnet, and host numbers as required. To prevent problems, make sure that duplicate or random network numbers do not exist in your organization. If you are planning to transition to IPv6, see Chapter 15, Transitioning From IPv4 to IPv6.

Designing Your IPv4 Addressing Scheme

After you have received 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, with each byte represented by xxx. "Host address" indicates the number of bytes 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 are dedicated to the host. The opposite is true for a class C network.

Table 5-1 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 and are always assigned by the InterNIC. 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 assigned to you.

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

Table 5-2 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, as explained in "Network Interfaces". 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, it must have its own unique IP number. Adding a second network interface changes the function of a machine from a host to a router, as explained in "Configuring Routers". If you add a second network interface to a host and disable routing, the host is then considered a multihomed host.

Each network interface has a device name, device driver, and 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 came with the network interface for configuration information.