Private Addresses

Private address are IP addresses that are available for everyone to use inside a private company, home, or office. RFC 1918, "Address Allocation for Private Internets," designates IP address ranges that have been blocked by IANA from use on the Internet.

Although you might think that private addresses are nonroutable, this is technically not true. These addresses work just fine in routing tables; they just do not exist ”or should not exist ”in routing tables on the Internet and thus cannot be routed anywhere . In the field, you will see these ranges used repeatedly because companies typically don't like to use real public Internet addresses inside their building or private networks. Instead, they prefer to use NAT to translate these nonroutable, private addresses to real public addresses that can be routed on the Internet. The list shown here displays the private IP address ranges that can be used:

• Class A ” 10.0.0.0 “10.255.255.255 mask 255.0.0.0

• Class B ” 172.16.0.0 “172.31.255.255 mask 255.240.0.0

• Class C ” 192.168.0.0 “192.168.255.255 mask 255.255.0.0