secondary ip

what is the use of secondary ips that we sometimes assigned to our interfaces.so it means that we can make 1 interface part of different subnets or the secondary ip address we are giving shud be from 1 subnet only. I am confused.do help me
thanx

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EdTheLad

Flexablility, the possibility to have 2 different net addresses on a router interface.

pannupandit

but how will use these separate subnet addresses. however at a time we can choose only 1 or both can be used.If both can be used at a time how we will use them

EdTheLad

pannupandit wrote:

but how will use these separate subnet addresses. however at a time we can choose only 1 or both can be used.If both can be used at a time how we will use them

Ok, an example would be if you had 2 corporate networks running rip you want to merge together,these 2 corpoarate networks both use 10.x.x.x network at some point within there large network.The available router interfaces to connect these 2 networks are already using a 200.1.1.x network.The problem you have now is a discontiguos network.Rip will see 10.x.x.x destination advertised from 2 different locations.By creating a secondary interface with a 10.x.x.x on all routers between the original 10.x.x.x networks you will remove the discontiguity( is that a word?).
Hard to explain without a diagram but maybe you get the idea.

mikej412

pannupandit wrote:

but how will use these separate subnet addresses. however at a time we can choose only 1 or both can be used.If both can be used at a time how we will use them

If you're not using DHCP and need to re-IP a network, but want the "old IPs" to still be able to access the network until you can get to them all, then you could use a Secondary Address. You may start by adding the "new IP" as the secondary to do the initial connectivity tests.

But when you use the secondary address feature you are running 2 different subnets on the same access layer switches. Some routing protocols like RIP will tell you that they see routing updates for "a different network" -- but I think there is a command to turn that off.

Its probably something that you shouldn't see much anymore in production networks -- sub-interfaces and VLANs give you a "cleaner" solution (or maybe a better designed solution).

Danman32

Another reason for secondary IP's on interfaces, especially on PCs being used as routers, is for IP-IP nat. Once NAT is properly configured, the secondary IP that is usually in the same subnet as the primary 'becomes' the IP of the host with the private IP address it is mapped to from the perspective of the public interface.