CIDR (Classless Inter-domain routing, Classless inter-domain Routing), DHCP, NAT

CIDR (Classless Inter-domain routing, Classless inter-domain Routing) is a way to create additional addresses on the Internet, which are provided to service providers (ISPs) and are assigned to customers by the ISP. CIDR combines routing to reduce the burden on Internet routers by enabling an IP address to represent the thousands of IP addresses of the primary backbone provider services.






Dhcp

Router Interface IP address is the default gateway



The benefits of broadcasting, the other server quickly retracted reserved for its IP






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Disadvantages of Nat
(1) Cannot process embedded IP address or port
NAT devices cannot translate IP addresses or port information that is embedded in the Application Data section. It can only translate the address information that is normally located in the IP header and the port information located in the TCP/UDP header (Figure 2) because the other party communicates with the address and port embedded in the received packet. This may cause connection failure, if the communication both sides are using the public network IP, this will not cause any problems, but if the embedded address and port is intranet, it is obvious that the connection will not be able to attack, as the beginning of the same reason. Some of the features of MSN Messenger Use this approach to pass IP and port information, which causes connectivity failures for client network applications after NAT devices.

To address the embedded address and port issues above, most of them are now implemented through a specific NAT editor (as shown in Figure 1), such as Windows 2000, which provides the respective NAT editors for FTP, PPTP, ICMP, and TCP on NetBIOS, in this way, Each application protocol requires a separate NAT editor, but the application protocol is so numerous that it is clear that this approach is unlikely to meet all requirements, so we still need a more perfect solution, as we'll talk about here.
(2) Cannot access the Internal network service from the public network
Because intranet is private IP, so can not directly from the public network access to internal network services, such as Web services, for this problem, we can establish a static mapping method to solve. For example, there is a static mapping, the 218.70.201.185:80 and 192.168.0.88:80 mapped, when the public network users to access the internal Web server, it first connected to the 218.70.201.185 : 80, then the NAT device passes the request to 192.168.0.88:80,192.168.0.88 to return the response to the NAT device, which is then passed on to the public network by the NAT device.
(3) Some applications, although using a port to send data, but the use of B port to receive, but the NAT device translation is not aware of this, it still establishes a port for a mapping, the result of the other response to the data to the B port, the NAT device can not find the relevant mapping entries and discards the packet.
(4) If Nat itself is located in another NAT, there will also be some problems, in order to save IP resources, now many ISPs in their internal set up NAT, and then the service provided to users, such as the Great Wall, friends and other ISPs are usually in the way of the metropolitan area Network to provide users with broadband Internet services.
(5) Some peer-to-peer applications cannot be performed after NAT
For pure Peer-to-peer applications without intermediate servers, if everyone is in a NAT device, the connection cannot be established. Because there is no intermediary server relay, the NAT device after the Peer-to-peer program on the NAT device is not a mapping bar, which means that the other party can not initiate a connection to you.

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