Introduction to IP Address unnumbered

Because the IP address is more nervous, so in the WAN link summary address allocation generally take the following 3 kinds of strategy: 1. Use VLSM 2. Use private address 3. Use the first variable-length subnet mask (VLSM) without a numbered IP (IP unnumbered) when using this, there must be no routing between classes, otherwise, there will be an error in the routing update information, and it is still using an IP address, except that it is more convenient for the exchange to make way for class routing. But it does not really add IP addresses. The second use of a common private IP has a disadvantage, because private addresses are not routed by Internet router, which can cause some restrictions, such as running ICMP ping for scheduling and remote telnet, and so on. Third, there are no restrictions on the first two, IP unnumbered The plain thing is to borrow an IP from another interface on the serial port (can be Ethernet or loopback), so it doesn't need its own address. and IP unnumbered can be used with classful routing protocols (inter-class routing) Its main purpose is to save valuable IP address resources. An interface cannot generate a route without an IP address, and it cannot forward a message. The so-called "Borrow IP address", its essence is: an interface is not configured IP address, but also want to use the interface. An IP address is borrowed from another IP address to enable the interface to be used properly. If the borrowed interface has more than one IP address, only the primary IP address can be borrowed.  If the borrowed interface does not have an IP address, the IP address of the borrowing interface is 0.0.0.0. It is to be noted that:

(1) The borrower shall not be an Ethernet interface.

(2) The address of the borrower's interface cannot be a borrowing address.

(3) The address of the borrower may be lent to multiple interfaces.

(4) The address of the loopback can be borrowed by other interfaces, but it cannot borrow the address of other interfaces.

(5) to lend "Their address of the LAN interface must meet one of 2:
-----The same primary network with different subnets, but the masks must be the same.
------Different main network default masks {that is, not partitioned primary network} Below is a configuration instance let's take a look. Suppose a company, headquartered in Beijing, has a branch office in Shenzhen and Shanghai. There is an office in Wuhan. Its network situation is shown below. R is the router of the headquarters, which is connected with the R1, R2, R3 of each branch and office by the Telephone Network (PSTN). R, R1, R2, R3 four routers have a serial port for dialing, and an Ethernet port for connecting to a local network. 1. Group Network Diagram 2. Configuration StepsConfiguring the Headquarters Router R: [quidway-ethernet1/0/0] IP address 172.16.10.1 255.255.255.0# Use Ethernet IP address: [quidway-serial2/0/0] IP Address unnumbered interface Ethernet 1/0/0[quidway-serial2/0/0] Link-protocol PPP# Configuration to the Shenzhen router R1 Ethernet network Segment routing: [Quidway] IP route-static 172.16.20.0 255.255.255.0 172.16.20.1# Configuration to the Shenzhen router R1 serial Port Interface route: [Quidway] IP route-static 172.16.20.1 255.255.255.255 serial2/0/0Configure the router R1 of Shenzhen branch: [quidway-ethernet1/0/0] IP address 172.16.20.1 255.255.255.0# Use Ethernet IP address: [quidway-serial2/0/0] IP Address unnumbered Ethernet 1/0/0[quidway-serial2/0/0] Link-protocol PPP# Configure routing to the R0 Ethernet segment of the Beijing headquarters router, which is the default route: [Quidway] IP route-static 0.0.0.0 0.0.0.0 172.16.10.1# Configuration to the Beijing router R serial Port interface route: [Quidway] IP route-static 172.16.10.1 255.255.255.255 serial2/0/0 3. Configuration Instructions The above configuration contains only the configuration associated with the borrowed IP address. Attention is drawn here to the configuration steps that the interface borrows from the IP address. At Beijing headquarters, router R must be configured with two static routes to access the host on the Ethernet of the Shenzhen router R1. The first static route is the network segment route to the Ethernet segment of the R1: the IP address (or the borrowed IP address) of the R1 serial port next hop. IP route-static 172.16.20.0 255.255.255.0 172.16.20.1 The second static route is the interface route to the R1 serial port, and the next hop is the R0 serial port. After IP route-static 172.16.20.1 255.255.255.255 serial0/0 Plus these two routes, R can correctly transfer IP packets accessing R1 Ethernet segment to R1. Similarly, two static routes are configured on the R1 to properly access the Ethernet segment of R. The first static route is the network segment route to the Ethernet segment of R: the IP address (or the borrowed IP address) of the serial port of the next hop to R. IP route-static 0.0.0.0 0.0.0.0 172.16.10.1 The second static route is the interface route to the R serial port, and the next hop is the R1 serial port. IP route-static 172.16.10.1 255.255.255.255 serial0/0 R2, R3 the configuration of reference R1.