How you assign IP addresses depends on whether your systems are connected to the public Internet. Systems connected to the Internet directly, rather than through a proxy server or other device doing NAT, must have unique, valid IP addresses, often termed legal addresses. This means you can't arbitrarily choose an address range for these systems. Instead, you need to obtain an address range from your ISP to ensure that you are using unique addresses (and that proper routing takes place). The number of addresses you need to obtain depends on how many hosts you will have on the public side of your proxy server or other NAT
device, if any. For example, assume you configure your network so that a proxy server sits between the router and all other hosts. You would need only three public addresses: one for each side of the router and one for the public side of the proxy server. The hosts on the private side of the proxy server can use private addresses.
If your network is not connected to the Internet, you could theoretically choose any network address range, including a public range in use by someone else, but you will not be able to connect your network to the Internet. You should, however, follow the convention of using one of the reserved address ranges for your private network (discussed previously in this chapter), because it will make life easier for you if you install NAT services. You won't have to re-address all of your hosts later if you decide to connect the network to the Internet — you simply provide some means of NAT through a router (such as Routing and Remote Access Service, RRAS, discussed later) or a proxy server.
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