Why have two ip addresses assigned to the same interface?

Lammle's latest Routing and Switching book in Chp 6 pg. 245 talks about having two ip addresses assigned to the same interface.

I can't replicate this command via Packet Tracer, however this sort of an example that he uses in the book.

int fa0/1 
ip address 192.168.100.10 255.255.255.0 
ip address 192.168.100.11 255.255.255.0 secondary

Lammle doesn't go into the reason why there is a need to have two ip addresses assigned to the same interface. Can anyone think of a reason?

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Architect192

Cisco IOS IP Configuration?Guide, Release?12.2 - Configuring IP Addressing [Cisco IOS Software Releases 12.2 Mainline] - Cisco

Assigning Multiple IP Addresses to Network Interfaces

Cisco IOS software supports multiple IP addresses per interface. You can specify an unlimited number of secondary addresses. Secondary IP addresses can be used in a variety of situations. The following are the most common applications:

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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.

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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.

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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.

cpartin

Legacy equipment / application support is one example. I have a site that uses this for that reason.

networker050184

Migration from one subnet to another is a big reason I've seen this as well. Throw the new network on and then devices can be migrated at the owners convenience. Once all devices are moved to the new subnet remove the old one.

tomtom1

I've seen it in the datacenter world as well.. Multiple subnets on the same VLAN.

Heero

In real applications, it is mostly used for IP address transitions. When we made customers renumber, we would make the change on our end of the connection and make the current address the secondary, and the new address the primary. Then we would copy any rDNS entries over, and then we would inform the customer that they need to change their IP address to the new IPs within X days. They can do it at their own convenience...you don't have to coordinate with them. Then when they let you know they finished, remove the old address (currently the secondary address).

There are plenty of other ways you could use them, but IP address transitions are the main thing I can think of where there isn't a different, better way to accomplish it.