Because IPv4 addresses are relatively scarce, each address typically costs money to use. You may not pay directly for an IP address, but your ISP has a limited supply of them and will use some mechanism to control their use. To allow multiple machines to run behind a limited supply of IP addresses, many networks use a block of private addresses as well as Network Address Translation (NAT) .
Supported by most routers, NAT processes all packets bound for the Internet from a private network, transforming the original IP addresses into an address that is reachable from the Internet, before sending the packets on. In effect, packets from the private network appear to be coming from the router running NAT (Figure 11-6). When packets arrive back at the router that corresponds to a connection already in progress, the NAT translates them into a form usable on the private network and sends them back to the original host.
You can tell if you are on a private network when your IP address is in one of the following ranges:
10.0.0.0 through 10.255.255.255
169.254.1.0 through 169.254.254.255
172.16.0.0 through 172.31.255.255
192.168.0.0 through 192.168.255.255
Using NAT creates two follow-on effects:
Since NAT relays only packets from the Internet that were requested by a computer on the private network, NAT acts as a basic firewall concealing the machines on the private network.
With no way to directly address a machine on a private network, it’s harder to set up a machine on the private network to act as a server. Some NAT routers let you forward specific ports from their external IP address to various machines on the private network.
You’ll need to consult the information about your router to be sure.