Approximately four billion addresses allowed in IPv4, three address blocks are reserved for the private network. These blocks outside the private network address is not routable, the hosts in the private network can not communicate directly with the public network. But through Network Address Translation (NAT), use these addresses can communicate with the host as the host has a total of addresses on the Internet.
The following table shows three of which are reserved for private network address blocks (RFC 1918):
| first name | Address range | The number of addresses | There are categories of description | Maximum CIDR address block |
|---|---|---|---|---|
| Block 24 | 10.0.0.0–10.255.255.255 | 16,777,216 | A class A | 10.0.0.0/8 |
| Block 20 | 172.16.0.0–172.31.255.255 | 1,048,576 | 16 consecutive Class B | 172.16.0.0/12 |
| Block 16 | 192.168.0.0–192.168.255.255 | 65,536 | Continuous 256 Class C | 192.168.0.0/16 |
Virtual Private Network
Main article: Virtual Private Network
Under normal circumstances, the router forwards the decision to the next hop address of the data packets based on the destination address of the data packet. Use a dedicated network address as the destination address of the packet routers usually can not be properly served the public, because the public do not have the router routing information that does not know how to be forwarded to the IP address. Therefore, we need a way to guide the next hop address to forward data packets to be transmitted and the true destination address separated. Thus it is a virtual private network, the IP packets are encapsulated in other packets to the public Internet to the router by the well, to the network device can handle the message data after the lifting of the inner packet, the packet may be continue forwarded to the destination address.
The data packet encapsulation process, the data packets may be encapsulated in IP packets, may also be used multiprotocol label switching protocol, the data packets forwarded by the guide other protocols. Encrypt data may be encapsulated to protect the data content.