Untitled

                [PC1]                  [PC2]                    [PC3]
                 |                      |                        |
         192.168.1.0/24         192.168.2.0/24           192.168.3.0/24
                 |                      |                        |
              Fa0/0                  Fa0/1                    Fa0/0
                 |                      |                        |
               [R1]------------------[R2]----------------------[R3]
              Fa0/1                Fa0/0  Fa0/1               Fa0/1
                 |                10.0.12.2 10.0.23.1         10.0.23.2
         10.0.12.0/30             (lien R1–R2)     (lien R2–R3)
                 |
              Fa0/2
             Internet