Information Technology Reference
In-Depth Information
IPv4 Private Addresses
Some network numbers within the IPv4 address space are reserved for private use. These
numbers are not routed on the Internet. Many organizations today use private addresses
in their internal networks with NAT to access the Internet. (NAT is covered later in this
chapter.) Private addresses are explained in RFC 1918,
Address Allocation for Private In-
ternets
, published in 1996. Private addresses were one of the first steps dealing with the
concern that the globally unique IPv4 address space would become exhausted. The avail-
ability of private addresses combined with NAT reduces the need for organizations to
carefully define subnets to minimize the waste of assigned, public, global IP addresses.
Key
To p i c
The IP network address space reserved for private internets is 10/8, 172.16/12, and
192.168/16. It includes one Class A network, 16 Class B networks, and 256 Class C net-
works. Table 8-12 summarizes private address space. Large organizations can use network
10.0.0.0/8 to assign address space throughout the enterprise. Midsize organizations can
use one of the Class B private networks 172.16.0.0/16 through 172.31.0.0/16 for IP ad-
dresses. The smaller Class C addresses, which begin with 192.168, support only up to 254
hosts each.
Ta b l e 8 -1 2
IPv4 Private Address Space
Class Type
Start Address
End Address
Class A
10.0.0.0
10.255.255.255
Class B
172.16.0.0
172.31.255.255
Class C
192.168.0.0
192.168.255.255
NAT
NAT devices convert internal IP address space into globally unique IP addresses. NAT was
originally specified by RFC 1631; the current specification is RFC 3022. Companies use
NAT to translate internal private addresses to public addresses and vice versa.
Key
To p i c
The translation can be from many private addresses to a single public address or from
many private addresses to a range of public addresses. When NAT performs many-to-one,
the process is called Port Address Translation (PAT) because different port numbers iden-
tify translations.
As shown in Figure 8-3, the source addresses for outgoing IP packets are converted to
globally unique IP addresses. The conversion can be configured statically, or it can dy-
namically use a global pool of addresses.
NAT has several forms:
Static NAT:
Maps an unregistered or private IP address to a registered IP address; it is
configured manually. It is commonly used to assign a network device with internal pri-
vate IP address a unique public address so that they can be accessed
■
