Designing an IPv4 Addressing Scheme
Note - For IPv6 address planning information, refer to Preparing an IPv6 Addressing Plan.
This section gives an overview IPv4 addressing to aid you in designing an
IPv4 addressing plan. For information on IPv6 addresses, see IPv6 Addressing Overview. For information on
DHCP addresses, see Chapter 12, About Solaris DHCP (Overview).
Each IPv4-based network must have the following:
A unique network number that is assigned by either an ISP, an IR, or, for older networks, registered by the IANA. If you plan to use private addresses, the network numbers you devise must be unique within your organization.
Unique IPv4 addresses for the interfaces of every system on the network.
A network mask.
The IPv4 address is a 32-bit number that uniquely identifies a network interface
on a system, as explained in How IP Addresses Apply to Network Interfaces. An IPv4 address is written in
decimal digits, divided into four 8-bit fields that are separated by periods. Each
8-bit field represents a byte of the IPv4 address. This form of
representing the bytes of an IPv4 address is often referred to as the
dotted-decimal format.
The following figure shows the component parts of an IPv4 address, 172.16.50.56.
Figure 2-1 IPv4 Address Format
- 172.16
Registered IPv4 network number. In class-based IPv4 notation, this number also defines the IP network class, Class B in this example, that would have been registered by the IANA.
- 50.56
Host part of the IPv4 address. The host part uniquely identifies an interface on a system on a network. Note that for each interface on a local network, the network part of the address is the same, but the host part must be different.
If you plan to subnet a class-based IPv4 network, you need to
define a subnet mask, or netmask, as explained in netmasks Database.
The next example shows of the CIDR format address 192.168.3.56/22
Figure 2-2 CIDR Format IPv4 Address
- 192.168.3
Network part, which consists of the IPv4 network number that is received from an ISP or IR.
- 56
Host part, which you assign to an interface on a system.
- /22
Network prefix, which defines how many bits of the address comprise the network number. The network prefix also provides the subnet mask for the IP address. Network prefixes are also assigned by the ISP or IR.
A Solaris-based network can combine standard IPv4 addresses, CIDR format IPv4 addresses, DHCP
addresses, IPv6 addresses, and private IPv4 addresses.
Designing Your IPv4 Addressing Scheme
This section describes the classes into which standard IPv4 address are organized. Though
the IANA no longer gives out class-based network numbers, these network numbers are
still in use on many networks. You might need to administer the address
space for a site with class-based network numbers. For a complete discussion of
IPv4 network classes, refer to Network Classes.
The following table shows the division of the standard IPv4 address 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 2-1 Division of the IPv4 Classes
Class |
Byte Range |
Network
Number |
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. The remaining three bytes have
a range from 0–255. The two numbers 0 and 255 are reserved. You
can assign the numbers 1–254 to each byte, depending on the network class
that was assigned to your network by the IANA.
The following table shows which bytes of the IPv4 address are assigned to
you. The table also shows the range of numbers within each byte that
are available for you to assign to your hosts.
Table 2-2 Range of Available IPv4 Classes
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 IANA |
1–254 |
1–254 |
C
|
192–223 |
Preassigned by IANA |
Preassigned by IANA |
1–254 |
IPv4 Subnet Number
Local networks with large numbers of hosts are sometimes divided into subnets. If
you divide your IPv4 network number into subnets, you need to assign a
network identifier to each subnet. You can maximize the efficiency of the IPv4
address space by using some of the bits from the host part
of the IPv4 address as a network identifier. When used as a network
identifier, the specified part of the address becomes the subnet number. You create
a subnet number by using a netmask, which is a bitmask that selects
the network and subnet parts of an IPv4 address. Refer to Creating the Network Mask for IPv4 Addresses for
details.
Designing Your CIDR IPv4 Addressing Scheme
The network classes that originally constituted IPv4 are no longer in use on
the global Internet. Today, the IANA distributes classless CIDR format addresses to its
registries around the world. Any IPv4 address that you obtain from an ISP
is in CIDR format, as shown in Figure 2-2.
The network prefix of the CIDR address indicates how many IPv4 addresses are
available for hosts on your network. Note that these host addresses are assigned
to interfaces on a host. If a host has more than one
physical interface, you need to assign a host address for every physical interface that
is in use.
The network prefix of a CIDR address also defines the length of
the subnet mask. Most Solaris 10 commands recognize the CIDR prefix designation of a
network's subnet mask. However, the Solaris installation program and /etc/netmask file require you
to set the subnet mask by using dotted decimal representation. In these two
cases, use the dotted decimal representation of the CIDR network prefix, as shown
in the next table.
Table 2-3 CIDR Prefixes and Their Decimal Equivalent
CIDR Network Prefix |
Available IP Addresses |
Dotted Decimal Subnet Equivalent |
|---|
/19 |
8,192 |
255.255.224.0 |
/20 |
4,096
|
255.255.240.0 |
/21 |
2,048 |
255.255.248.0 |
/22 |
1024 |
255.255.252.0 |
/23 |
512 |
255.255.254.0 |
/24 |
256 |
255.255.255.0 |
/25 |
128 |
255.255.255.128 |
/26 |
64 |
255.255.255.192 |
/27 |
32 |
255.255.255.224 |
For more information on CIDR addresses, refer to the following sources:
Using Private IPv4 Addresses
The IANA has reserved three blocks of IPv4 addresses for companies to use
on their private networks. These addresses are defined in RFC 1918, Address Allocation for Private Internets. You can
use these private addresses, also known as 1918 addresses, for systems on local networks
within a corporate intranet. However, private addresses are not valid on the Internet.
Do not use them on systems that must communicate outside the local network.
IPv4
Address Range |
netmask |
|---|
10.0.0.0 - 10.255.255.255 |
10.0.0.0 |
172.16.0.0 - 172.31.255.255 |
172.16.0.0 |
192.168.0.0 - 192.168.255.255 |
192.168.0.0 |
How IP Addresses Apply to Network Interfaces
To connect to the network, a system must have at least one
physical network interface. Each network interface must have its own unique IP address. During Solaris
installation, you must supply the IP address for the first interface that the
installation program finds. Usually that interface has the name device-name0, for example eri0
or hme0. This interface is considered the primary network interface.
If you add a second network interface to a host, that interface
also must have its own unique IP address. When you add the second
network interface, the host then becomes multihomed. By contrast, when you add a second
network interface to a host and enable IP forwarding, that host becomes a
router. See Configuring an IPv4 Router for an explanation.
Each network interface has a device name, a device driver, and an
associated device file in the /devices directory. The network interface might have a device
name such as eri or smc0, which are device names for two commonly
used Ethernet interfaces.
For information and tasks related to interfaces, refer to Chapter 6, Administering Network Interfaces (Tasks).
Note - This book assumes that your systems 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.
