Jack,

I think you are talking about secondry Ip addresses here. Am I correct? If yes, secondry ip addresses are primarily used for temporary IP address allocation to the users and also during the network ip address range migration. If your primary subnet has all the host addresses assigned already and you need to have more hosts and there is not another interface on the router to use, then secondary address is the best solution available to you.

There are some things to keep in mind about using secondary addressing but I do not think that they will impact you, given the description of your environment: if using secondary addressing and there is more than one router with interfaces onto that network segment all routers should be careful to use the same subnet as the primary address; if running a routing protocol on an interface with secondary addressing the router will source its routing update from the primary address and not the secondary address; if running a routing protocol on an interface with secondary addressing the router will form neighbor relationships and/or adjacencies with devices on the primary address but not the secondary address.

If using secondary addressing and a device in the primary subnet wants to communicate with a device in the secondary subnet they might communicate with each other directly, but they might send all their communications through the router (sending to their default gateway) and this can increase the amount of traffic going through the router interface.

HTH,Please rate if it does.

-amit singh