Lab 32: Configuring Static NAT and DLSw-Part I

NAT and RFC 1918

Many of the benefits of NAT are derived from the goal of conserving public IP address space. As you can see from the preceding example, NAT allows you to have private addressing while accessing services outside your routing domain. RFC 1918, "Address Allocation for Private Internets," sets aside ranges of addresses that ISPs will not forward. This allows for the design of many internetworks capable of accessing Internet services without having to worry about having registered addresses. Specifically, RFC 1918 reserves the IP address ranges listing in Table 15-2.

Table 15-2. IP Address Range for RFC 1918

IP Address Class	IP Address Range
Class A	10.0.0.0 to 10.255.255.255
Class B	172.16.0.0 to 172.31.255.255
Class C	192.168.0.0 to 192.168.255.255

NOTE

When using private address space, use it wisely. Unfortunately, many network designs have implemented using the 10s network with 24-bit mask on point-to-point links, 16-bit masks on Ethernet segments, and so on. Just because the address space is vast, don't let this lead to sloppy IP address schemes. Remember that proper subnetting helps with route summarization and route propagation, both of which can have a significant impact on large internetworks. Many internetworks are also built around the 10s subnet. In the few years that this RFC has been out, many networks have merged and have had address overlaps on the 10s subnet, while the thousands of networks that reside on the 172.16.0.0 remain unused. My point again is to use this space wisely; treat it as you would if the address spaces were registered. Be original in your design. You will find that the little extra time you take in proper IP design will benefit you immensely as your internetwork scales and grows together with other internetworks.