Scaling the Address Space with Network Address Translation
With Network Address Translation (NAT), you can expand your IP address space by deploying so-called private addresses and translating them into publicly registered addresses. NAT can be a viable option in slowing address space depletion, and using it might be more feasible than redesigning the network with VLSM or obtaining new public addresses with your ISP or Internet registry (American Registry for Internet Numbers, if you are in North or South Ajnerica).
Private addresses are blocks of the IP address space that the Internet community has set aside for use by networks that do not communicate with the public Internet. The address blocks are defined in RFC 1918 and include
• 172.16.0.0 through 172.31.255.255 (172.16.0.0/12)
• 192.168.0.0 through 192.168.255.255 (192.168.0.0/16)
Any organization may freely deploy these addresses without notifying the Internet registry. Thus, multiple organizations can use these addresses, each in their private networks, with the understanding that the public Internet does not route traffic to or from these addresses. This might be applicable for hosts that do not need to communicate over the Internet and have no intention to communicate over the Internet in the future (private computer labs arc an example). These addresses are deployed within the organization just as any ordinary IF address space, and the same subnetting rules and VLSM techniques apply to these private addresses as to normal public addresses.
Public addresses, on the other hand, are administered by the Internet registry and are routable by the Internet. Every public address is unique (no two hosts on the Internet have the same IP address) and has a registered owner if it's in use. A host addressed with a public address can communicate with hosts both inside the organization and outside on the Internet.
If everyone could have as many public addresses as they want, there would be no need to use private addresses. But the Internet has a finite number of addresses, and getting a share of the public addresses can become difficult as more scrutiny and tighter control are used to determine who gets them. Private addresses are readily available for use, bul they come with the big disadvantage that they cannot be used to communicate over the Internet. What you want is the best of both worlds: use of the private address space and the ability to communicate over the Internet. This requires a way to translate addresses as they flow between the private and public domains—which is where NAT comes into play.
This section continues with discussions on
• Translating Private Addresses into Public Addresses
• Configuring NAT
• Creating a Pool of Discontiguous Addresses
• Configuring Static NAT
• Special Applications and NAT
• More Important Points on NAT
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