NAT Operation

Types of NAT

There are three types of NAT translation:

• Static address translation (static NAT) - One-to-one address mapping between local and global addresses.

• Dynamic address translation (dynamic NAT) - Many-to-many address mapping between local and global addresses.

• Port Address Translation (PAT) - Many-to-one address mapping between local and global addresses. This method is also known as overloading (NAT overloading).

Static NAT

Static NAT uses a one-to-one mapping of local and global addresses. These mappings are configured by the network administrator and remain constant.

In the figure, R2 is configured with static mappings for the inside local addresses of Svr1, PC2, and PC3. When these devices send traffic to the Internet, their inside local addresses are translated to the configured inside global addresses. To outside networks, these devices have public IPv4 addresses.

Static NAT is particularly useful for web servers or devices that must have a consistent address that is accessible from the Internet, such as a company web server. It is also useful for devices that must be accessible by authorized personnel when offsite, but not by the general public on the Internet. For example, a network administrator from PC4 can SSH to Svr1’s inside global address (209.165.200.226). R2 translates this inside global address to the inside local address and connects the administrator’s session to Svr1.

Static NAT requires that enough public addresses are available to satisfy the total number of simultaneous user sessions.