Module 1: Scaling IP Addresses / Private addressing

Module 1: Scaling IP Addresses
1.1.1 Private Addressing
RFC 1918 sets aside the following three blocks of private IP addresses:

• 1 Class A address
• 16 Class B addresses
• 256 Class C addresses

These addresses are for private, internal network use only. Packets containing these addresses are not routed over the Internet.
Public Internet addresses must be registered by a company with an Internet authority, for example, American Registry for Internet Numbers (ARIN) or Réseaux IP Européens (RIPE), the Regional Internet Registry responsible for Europe and North Africa. These public Internet addresses can also be leased from an ISP. Private IP addresses are reserved and can be used by anyone. That means two networks, or two million networks, can each use the same private address. A router should never route RFC 1918 addresses. ISPs typically configure the border routers to prevent privately addressed traffic from being forwarded.
NAT provides great benefits to individual companies and the Internet. Before NAT, a host with a private address could not access the Internet. Using NAT, individual companies can address some or all of their hosts with private addresses and use NAT to provide access to the Internet.