Cisco IOS supports multiple IP Addresses on an interface. You can configure one Primary IP Address and multiple Secondary IP Addresses per Router interface. There is no limit on the number of Secondary IP Addresses that can be configuring per Router interface. Configuring multiple IP Addresses on your cisco routers can sometime help where you have multiple subnets having one physical Router interface. Secondary IP addresses can be used in a variety of situations. The following are the most common applications:
1. There might not be enough host addresses for a particular network segment. For example, suppose your subnetting allows up to 254 hosts per logical subnet, but on one physical subnet you must have 300 host addresses. Using secondary IP addresses on the routers or access servers allows you to have two logical subnets using one physical subnet.
2. Many older networks were built using Level 2 bridges, and were not subnetted. The judicious use of secondary addresses can aid in the transition to a subnetted, router-based network. Routers on an older, bridged segment can easily be made aware that many subnets are on that segment.
3. Two subnets of a single network might otherwise be separated by another network. You can create a single network from subnets that are physically separated by another network by using a secondary address. In these instances, the first network is extended, or layered on top of the second network. Note that a subnet cannot appear on more than one active interface of the router at a time.
Configure Multiple IP addresses on Cisco Router Interface:
You can configure the IP Addresses from different subnets and from different networks. From the Router Interface configuration mode, configure the primary IP Address and secondary IP Addresses. To configure multiple IP Addresses,
Configure primary IP Address on Router Interface:
Router# config t
Router(config)# int fa0/1
Router(config-if)# ip address 192.168.1.1 255.255.255.0
Configure Secondary IP Address on Router Interface:
Now, add the required secondary IP Addresses onto the same interface of the Router.
Router(config-if)# ip address 172.16.2.1 255.255.255.0 secondary
This adds the additional IP Address onto the interface. You can add multiple IP addresses using above command. In the command above, the keyword “secondary” at the end defines the additional IP Address as a secondary IP Address.
Filed Under: Cisco General
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