IP Routing

Subnet Masks

Looking closely at Figure 2-2, we see that there is no field for subnet masks in RIP. Let’s say that router SantaFe received an update with the following routes in the IP address field:

192.100.1.48
192.100.1.64
192.100.2.0
10.0.0.0

And let’s say that SantaFe has the following configuration:

hostname SantaFe
!
interface Ethernet 0
ip address 192.100.1.17 255.255.255.240
!
interface Ethernet 1
ip address 192.100.1.33 255.255.255.240
!
router rip
network 192.100.1.0
network 192.100.2.0

How would the router associate subnet masks with these routes?

• If the router has an interface on a network number received in an update, it would associate the same mask with the update as it does with its own interface. Consequently, RIP does not permit Variable Length Subnet Masks (VLSM).

• If the router does not have an interface on the network number received in an update, it would assume a natural mask for the network number.

SantaFe’s routing table would look like this:

SantaFe>sh ip route
...
Gateway of last resort is not set

R       10.0.0.0 [120/1] via 192.100.1.18, 0:00:11, Ethernet0
R       192.100.2.0 [120/1] via 192.100.1.18, 0:00:11, Ethernet0
     192.100.1.0/16 is subnetted, 4 subnets
C       192.100.1.16 is directly connected, Ethernet0
C       192.100.1.32 is directly connected, Ethernet1
R       192.100.1.48 [120/1] via 192.100.1.18, 0:00:11, Ethernet0
R       192.100.1.64 [120/1] via 192.100.1.18, 0:00:11, Ethernet0

SantaFe represents 192.100.1.48 and 192.100.1.64 with a 28-bit mask even though the subnet mask was ...