NAT方面的問題：我使用兩台2501做的實驗一台為R1是連接外網和內網的；另一台為R2是連接內網的。兩台路由器通過串口相連。可是我在R1上配置動態NAT後，在R1上PING不同外網網關，在R2上也PING不通外網的網關，但是與R2通過以太口相連的PC機卻能PING通。請問為什麼。希望大家解答。

R1的配置清單：

!
version 12.1
no service single-slot-reload-enable
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Router
!
logging rate-limit console 10 except errors
enable secret 5 $1$aAp1$E.uusF93GanQDeyJ7BdKt/
enable password 1234
!
ip subnet-zero
no ip finger
no ip domain-lookup
!
cns event-service server
!
!
!
!
!
interface Ethernet0
ip address 220.113.3.224 255.255.255.0
ip nat outside
!
interface Serial0
bandwidth 10000000
ip address 192.168.12.12 255.255.255.0
ip nat inside
shutdown
!
interface Serial1
bandwidth 10000000
ip address 192.168.12.12 255.255.255.0
ip nat inside
clockrate 4000000
!
ip kerberos source-interface any
ip nat pool lab2 220.113.3.224 220.113.3.225 netmask 255.255.255.0
ip nat inside source list 1 pool lab2
ip classless
ip route 0.0.0.0 0.0.0.0 220.113.3.1
ip route 192.168.1.0 255.255.255.0 192.168.12.11
no ip http server
!
access-list 1 permit any
!
!
line con 0
logging synchronous
transport input none
line aux 0
line vty 0 4
login
!
end


R2配置清單：

!
version 12.1
no service single-slot-reload-enable
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Router
!
logging rate-limit console 10 except errors
enable secret 5 $1$6SeG$soDYqvwWI4tZbXoMSI1tm.
enable password 1234
!
ip subnet-zero
no ip finger
no ip domain-lookup
!
cns event-service server
!
!
!
!
!
interface Ethernet0
ip address 192.168.1.1 255.255.255.0
!
interface Serial0
bandwidth 10000000
ip address 192.168.12.11 255.255.255.0
shutdown
no fair-queue
!
interface Serial1
bandwidth 10000000
ip address 192.168.12.11 255.255.255.0
!
ip kerberos source-interface any
ip classless
ip route 0.0.0.0 0.0.0.0 192.168.12.12
no ip http server
!
!
!
line con 0
logging synchronous
transport input none
line aux 0
line vty 0 4
password 1234
login
!
end