significance of secondary ip address on a single interface.

[Sharanjeet_Kumar]

Hello All,

I just know that on a cisco router we can assign 2 IP addresses on a single interface.

first one is primary and second one is as a secondary.

my question is what is the significance of configuring the secondary IP address????

Any help is appreciated!!!!!!  

[marce1000]

 - FYI : https://learningnetwork.cisco.com/s/question/0D53i00000Kt5yrCAB/secondary-ip-address

 M.

[paul driver]

Hello
They could be used to extend your existing ip range for a particular network or when you wish to migrate from a certain ip subnet to another?

[MHM Cisco World]

ccie secondary IP - Google Search

I recommend to watch this video 

[Sharanjeet_Kumar]

Hi @MHM Cisco World,

i am not getting from this video.

as i asked for the secondary ip address on physical interface.

has it any significance to giving a secondary as well primary ip to a single physical interface.

[MHM Cisco World]

first as My colleagues answer you that you need it for network expand.
network expand ??
Yes if you have wireless network then it normally that it you have more than 255 /24 users for subnet. 
now why secondary I can only increase the subnet mask from /24 to be /23. ?
Yes you can both incease the subnet and add secondary IP, but 
if you have already large network it is better to change subnet mask which lead you to check all subnet in your network to prevent conflict or you can easily add new subnet (secondary IP) with same subnet mask but different network ID.

that why we see a lot of secondary subnet in Wireless network. 
thanks a lot. 

[Richard Burts]

When we think about a physical interface (and not considering any possible sub-interfaces) we generally assume that the interface will connect to a single subnet/network and will have a single IP address configured. But sometimes we might want to change that assumption and have the physical interface connect to more than one subnet/network. As explained by my colleagues that might be the case if the original subnet is small (perhaps the connected subnet is 10.10.10.0 255.255.255.248 (which gives you 6 usable addresses)) and you have more than 6 devices. By configuring a secondary address you make more addresses available through this interface. Or it might be the case that the existing subnet is 10.10.10.0/24 and you want to change the subnet to 172.16.1.0/24. By configuring the secondary address you make the interface function for both subnets at the same time, and when the address conversion is complete then you remove the secondary address and make the new subnet address be the primary address.