Network Address Translation (NAT)
Firstly, we could use Network Address Translation (NAT) which is a router function. IP addresses of IP datagrams are 'replaced' at the boundary of a private network, this enables the hosts on a private network to communicate with the Internet. NAT then connects private networks by replacing IP address-port pairs with another 'internet facing' IP address-port pair.

In short, NAT serves three main purposes:

Provides a type of firewall by hiding internal IP addresses
Enables a company to use more internal IP addresses. Since they're used internally only, there's no possibility of conflict with IP addresses used by other companies and organisations
Allows a company to combine multiple ISDN connections into a single Internet connection.

Private IP networks are not directly connected to the Internet and IP addresses are arbitrarily assigned, so they are not guaranteed to be unique.

The advantages of NAT is the added security since it hides the 'real' IP address of the host. It allows for more machines than IP addresses and there is no need to be allocated a range of addresses from the Internet Service Provider (ISP), just one IP address is used for all devices. You can also change the addresses of devices in the local network without notifying the outside world. Finally, you can change the ISP without changing the addresses of devices in the local network.

However, there are disadvantages. Firstly, performance is affected when you modify the IP header in order to change the IP address. This is because NAT routers need to recalculate the IP header checksum and modifying port number requires NAT routers to recalculate the TCP checksum. NAT doesn't consider fragmentation. This is because a datagram is fragmented before it reaches a NAT device, it must not be assigned a different IP address or different port numbers for each of the fragments. NAT also destroyed the universal end-to-end reachability of hosts on the Internet. This means that a host in the public Internet cannot initiate communication to a host in a private network. The problem is even worse when two hosts that are in a private network need to communicate with each other.

	Created by Liam-Beckwith over 8 years ago

This set of notes will expand a little further into topologies, looking a centralisation and decentralisation. We will also look into what is contained with a data packet and the concept of Network Address Translation (NAT) before finishing off with a brief overview of IP addresses.