By default, when using the network command and an IPv4 network address, such as 172.16.0.0, all interfaces on the router that belong to that classful network address are enabled for EIGRP. However, there may be times when the network administrator does not want to include all interfaces within a network when enabling EIGRP. For example, in Figure 1, assume that an administrator wants to enable EIGRP on R2, but only for the subnet 192.168.10.8 255.255.255.252, on the S0/0/1 interface.
To configure EIGRP to advertise specific subnets only, use the wildcard-mask option with the network command:
Router(config-router)# network network-address [wildcard-mask]
Think of a wildcard mask as the inverse of a subnet mask. The inverse of subnet mask 255.255.255.252 is 0.0.0.3. To calculate the inverse of the subnet mask, subtract the subnet mask from 255.255.255.255 as follows:
0. 0. 0. 3 Wildcard mask
Figure 2 continues the EIGRP network configuration of R2. The network 192.168.10.8 0.0.0.3 command specifically enables EIGRP on the S0/0/1 interface, a member of the 192.168.10.8 255.255.255.252 subnet.
Some IOS versions also let you enter the subnet mask instead of a wildcard mask. Figure 3 shows an example of configuring the same S0/0/1 interface on R2, but this time using a subnet mask in the network command. However, if the subnet mask is used, the IOS converts the command to the wildcard-mask format within the configuration. This is verified in the show running-config output in Figure 3.
Use the Syntax Checker in Figure 4 to configure the EIGRP network commands for router R3.