EIGRP Auto Summarization

Router(config-router)#no auto-summary	Turns off the auto-summarization feature. Networks are summarized at the classful boundary by default
Router(config)#int fa 0/0
Router(config-if)ip summary-address eigrp 100 10.10.0.0 255.255.0.0	Enables manual summarization on this specific interface for the given address and mask

Caution:

EIGRP automatically summarizes networks at the classful boundary. A poorly designed network with discontiguous subnets could have problems with connectivity if the summarization feature is left on. You could have two routers advertise the same network172.16.0.0/16when in fact the intention is for the routers to advertise two different networks172.16.10.0/24 and 172.16.20.0/24.

Recommended practice is that you turn off automatic summarization, use the ip summary-address command, and summarize manually what you need to.