Identifying the Differences Between Public and Private Networks
| IP addressing involves many considerations, not least important of which are public and private networks. A public network is a network to which anyone can connect. The best, and perhaps only pure, example of such a network is the Internet. A private network is any network to which access is restricted. A corporate network or a network in a school are examples of private networks.
The main difference between public and private networks, apart from the fact that access to a private network is tightly controlled and access to a public network is not, is that the addressing of devices on a public network must be considered carefully, whereas addressing on a private network has a little more latitude. As already discussed, in order for hosts on a network to communicate by using TCP/IP, they must have unique addresses. This number defines the logical network each host belongs to and the host's address on that network. On a private network with, say, three logical networks and 100 nodes on each network, addressing is not a particularly complex task. On a network on the scale of the Internet, however, addressing is very complex. If you are connecting a system to the Internet, you need to get a valid registered IP address. Most commonly, you would obtain this address from your ISP. Alternatively, for example, if you wanted a large number of addresses, you could contact the organization responsible for address assignment in your geographical area. You can determine who the regional numbers authority for your area is by visiting the IANA website. Because of the nature of their business, ISPs have large blocks of IP addresses that they can assign to their clients. If you need a registered IP address, getting one from an ISP will almost certainly be a simpler process than going through a regional numbers authority. Some ISPs' plans actually include blocks of registered IP addresses, working on the principle that businesses are going to want some kind of permanent presence on the Internet. Of course, if you discontinue your service with the ISP, you will no longer be able to use the IP address they provided. Private Address RangesTo provide flexibility in addressing and to prevent an incorrectly configured network from polluting the Internet, certain address ranges are set aside for private use. These address ranges are called private ranges because they are designated for use only on private networks. These addresses are special because Internet routers are configured to ignore any packets they see that use these addresses. This means that if a private network "leaks" onto the Internet, it won't make it any farther than the first router it encounters. Three ranges are defined in RFC 1918one each from Classes A, B, and C. You can use whichever range you want, although the Class A and Class B address ranges offer more addressing options than does Class C. The address ranges are defined in Table 5.4.
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