IPv4 101

Setting an IP Address

R(config)#ip address IP-ADDR NETMASK
Any combination of IP-ADDR and NETMASK can be used as long as the host portion of the address is not all zeros. One exception is allowed, when using a /31 mask. By default, the router will accept combinations that result in a subnet portion of the address with all zeros, but this behavior can be disabled using:
R(config)#no ip subnet-zero

Secondary IP Addresses

Multiple IP Addresses can be configured on an interface using the secondary keyword:
R(config-if)# ip address IP-ADDR NETMASK secondary

IP Unnumbered

This feature allows a router to use the IP address of another interface instead of using a new IP Address. It can only be configured on point-to-point interfaces
R(config-if)# ip unnumbered INTERFACE
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Setting an IP Address
Secondary IP Addresses
IP Unnumbered