NAT Types
NAT translates the IP address in an IP datagram header into another IP address, allowing users on private networks to access public networks. You can translate IP addresses by using dynamic NAT or static NAT based on whether the rules for translating IP addresses or port numbers are determined in advance.
Dynamic NAT
The address pool used by dynamic NAT stores a set of public IP addresses. When accessing public networks, intranet users select available public IP addresses from the address pool for NAT. For details, see Dynamic NAT in Address Pool Mode.
If there is only one public IP address available, it is set to the outbound interface IP address of the NAT device. When accessing public networks, intranet users use the outbound interface IP address of the NAT device and different port numbers for address translation. This simple mapping mode is called dynamic NAT in Easy IP mode. For details, see Dynamic NAT in Easy IP Mode.
Static NAT
- For example, as shown in Figure 5-3, when an intranet server needs to proactively provide services for extranet users, the NAT server performs only address translation. That is, when the source IP address of a packet matches the private IP address in a NAT server rule, the source IP address of the packet is translated regardless of the port number.
- As shown in Figure 5-4, however, NAT static translates both IP addresses and port numbers when intranet users proactively access extranet hosts. That is, the source IP address and port number of a packet are translated only when they match the private IP address and port number of a NAT static rule at the same time.
