注意：/31位掩码只能用在点对点链路上

topo:  R1====R2=====R3

R1#sh run
hostname R1
interface Serial1/1
ip address 192.168.1.2 255.255.255.254
router rip
version 2
network 192.168.1.0
no auto-summary
==============================================
R2#show running-config
hostname R2
interface Serial1/0
ip address 192.168.1.3 255.255.255.254
serial restart-delay 0
!
interface Serial1/1
ip address 192.168.2.1 255.255.255.0
serial restart-delay 0

router rip
version 2
network 192.168.1.0
network 192.168.2.0
no auto-summary
===============================================
R3#sh running-config
hostname R3
interface Serial1/0
ip address 192.168.2.2 255.255.255.0
serial restart-delay 0
router rip
version 2
network 192.168.2.0
no auto-summary
================================================
实验验证:
R1#ping 192.168.1.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 48/77/124 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
================================================
R3#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
   D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
   N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
   E1 - OSPF external type 1, E2 - OSPF external type 2
   i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
   ia - IS-IS inter area, * - candidate default, U - per-user static route
   o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
    192.168.1.0/31 is subnetted, 1 subnets
R    192.168.1.2 [120/1] via 192.168.2.1, 00:00:05, Serial1/0
C 192.168.2.0/24 is directly connected, Serial1/0