/31位掩码应用

/31位掩码应用：

R1#sh run

interface Serial1/1
ip address 192.168.1.2 255.255.255.254
router rip
version 2
network 192.168.1.0
--------------------------------
R2#show run
hostname R2
interface Serial1/0
ip address 192.168.1.3 255.255.255.254
!
interface Serial1/1
ip address 192.168.2.1 255.255.255.0

router rip
version 2
network 192.168.1.0
network 192.168.2.0

---------------------------------------
R3#sh run

interface Serial1/0
ip address 192.168.2.2 255.255.255.0

router rip
version 2
network 192.168.2.0
--------------------------------------
r1#ping 192.168.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 52/90/128 ms

R3#ping 192.168.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 120/132/144 ms

实验完成

后继：

This option provides the same address space savings as
   using a 31-bit subnet mask, but may only be used in links using PPP
   encapsulation [RFC1332].  The use of host addresses allows for the
   assignment of IP addresses belonging to different networks at each
   side of the link, causing link and network management not to be
   straight forward.
----
 the loss of functionality of a directed
   broadcast may actually be seen as a beneficial side effect, as it
   slightly enhances the network's resistance to a certain class of DoS
   Attacks [RFC2644, SMURF].

When a device wants to reach all the hosts on a given (remote, rather
   than directly connected) subnet, it may set the packet's destination
   address to the link's subnet broadcast address.  This operation is
   not possible for point-to-point links with a 31-bit prefix.

so use default route can solve this problom

R1
!
interface Serial1/1
 ip address 192.168.1.7 255.255.255.254
 encapsulation ppp
 serial restart-delay 0

router rip
 version 2
 network 192.168.1.0
!
ip classless
ip route 0.0.0.0 0.0.0.0 192.168.1.3

R2
!
interface Serial1/0
 ip address 192.168.1.3 255.255.255.254
 encapsulation ppp
 serial restart-delay 0

!
router rip
 version 2
 network 192.168.1.0
 network 192.168.2.0

R3
interface Serial1/0
 ip address 192.168.2.2 255.255.255.0
router rip
 version 2
 network 192.168.2.0

r3#ping 192.168.1.7

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.7, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/56/92 ms

-------------

补充：

r1(config)#int s1/1
r1(config-if)#ip add 192.168.1.4 255.255.255.254
r1(config-if)#ip add 192.168.1.0 255.255.255.254

r3#ping 192.168.1.0

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.0, timeout is 2 seconds:
!!!!!
r3#ping 192.168.1.4

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.4, timeout is 2 seconds:
!!!!!