NAT Overload

NAT overloading (sometimes called Port Address Translation or PAT) maps multiple private IP addresses to a single public IP address or a few addresses. This is what most home routers do. Your ISP assigns one address to your router, yet several members of your family can simultaneously surf the Internet.
With NAT overloading, multiple addresses can be mapped to one or to a few addresses because each private address is also tracked by a port number. When a client opens a TCP/IP session, the NAT router assigns a port number to its source address. NAT overload ensures that clients use a different TCP port number for each client session with a server on the Internet. When a response comes back from the server, the source port number, which becomes the destination port number on the return trip, determines to which client the router routes the packets. It also validates that the incoming packets were requested, thus adding a degree of security to the session.
NAT overload attempts to preserve the original source port. However, if this source port is already used, NAT overload assigns the first available port number starting from the beginning of the appropriate port group 0-511, 512-1023, or 1024-65535. When there are no more ports available and there is more than one external IP address configured, NAT overload moves to the next IP address to try to allocate the original source port again. This process continues until it runs out of available ports and external IP addresses.

ifferences Between NAT and NAT Overload

The differences between NAT and NAT overload will help your understanding. NAT generally only translates IP addresses on a 1:1 correspondence between publicly exposed IP addresses and privately held IP addresses. NAT overload modifies both the private IP address and port number of the sender. NAT overload chooses the port numbers seen by hosts on the public network.
NAT routes incoming packets to their inside destination by referring to the incoming source IP address given by the host on the public network. With NAT overload, there is generally only one or a very few publicly exposed IP addresses. Incoming packets from the public network are routed to their destinations on the private network by referring to a table in the NAT overload device that tracks public and private port pairs. This is called connection tracking.

Benefits and Drawbacks of Using NAT
NAT provides many benefits and advantages. However, there are some drawbacks to using NAT, including the lack of support for some types of traffic.

The benefits of using NAT include the following:
NAT conserves the legally registered addressing scheme by allowing the privatization of intranets. NAT conserves addresses through application port-level multiplexing. With NAT overload, internal hosts can share a single public IP address for all external communications. In this type of configuration, very few external addresses are required to support many internal hosts.
NAT increases the flexibility of connections to the public network. Multiple pools, backup pools, and load-balancing pools can be implemented to ensure reliable public network connections.
NAT provides consistency for internal network addressing schemes. On a network without private IP addresses and NAT, changing public IP addresses requires the renumbering of all hosts on the existing network. The costs of renumbering hosts can be significant. NAT allows the existing scheme to remain while supporting a new public addressing scheme. This means an organization could change ISPs and not need to change any of its inside clients.
NAT provides network security. Because private networks do not advertise their addresses or internal topology, they remain reasonably secure when used in conjunction with NAT to gain controlled external access. However, NAT does not replace firewalls.

However, NAT does have some drawbacks.
The fact that hosts on the Internet appear to communicate directly with the NAT device, rather than with the actual host inside the private network, creates a number of issues. In theory, a single globally unique IP address can represent many privately addressed hosts. This has advantages from a privacy and security point of view, but in practice, there are drawbacks.
The first disadvantage affects performance. NAT increases switching delays because the translation of each IP address within the packet headers takes time. The first packet is process-switched, meaning it always goes through the slower path. The router must look at every packet to decide whether it needs translation. The router needs to alter the IP header, and possibly alter the TCP or UDP header. Remaining packets go through the fast-switched path if a cache entry exists; otherwise, they too are delayed.

Many Internet protocols and applications depend on end-to-end functionality, with unmodified packets forwarded from the source to the destination. By changing end-to-end addresses, NAT prevents some applications that use IP addressing. For example, some security applications, such as digital signatures, fail because the source IP address changes. Applications that use physical addresses instead of a qualified domain name do not reach destinations that are translated across the NAT router. Sometimes, this problem can be avoided by implementing static NAT mappings.
End-to-end IP traceability is also lost. It becomes much more difficult to trace packets that undergo numerous packet address changes over multiple NAT hops, making troubleshooting challenging. On the other hand, hackers who want to determine the source of a packet find it difficult to trace or obtain the original source or destination address.
Using NAT also complicates tunneling protocols, such as IPsec, because NAT modifies values in the headers that interfere with the integrity checks done by IPsec and other tunneling protocols.
Services that require the initiation of TCP connections from the outside network, or stateless protocols such as those using UDP, can be disrupted. Unless the NAT router makes a specific effort to support such protocols, incoming packets cannot reach their destination. Some protocols can accommodate one instance of NAT between participating hosts (passive mode FTP, for example), but fail when both systems are separated from the Internet by NAT.


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