In EIGRP the auto-summary is by default enabled, means if you are just configuring the interfaces with class A, Class B or class C ip address then no matter what netmask you've applied on interface or under "router eigrp" process.
the router will advertise the address as per the mask of that class.
R1(1.1.1.1)----------------(1.1.1.2)R2
now R1 is having loopback address
R1#sh ip int brief | in Loop
Loopback1 10.1.1.1 YES manual up up
Loopback10 200.1.1.1 YES manual up up
Loopback100 60.1.1.1 YES manual up up
R1#
R1#sh running-config | sec router
router eigrp 10
network 1.0.0.0
network 10.0.0.0
network 200.1.1.0
auto-summary
R1#
it doesn't matter how you are defining the network inside "router eigrp "
R2#sh ip route eigrp
1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 1.0.0.0/8 is a summary, 00:17:46, Null0
20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 20.0.0.0/8 is a summary, 00:17:46, Null0
D 10.0.0.0/8 [90/409600] via 1.1.1.1, 00:00:24, Ethernet0/0
D 200.1.0.0/16 [90/409600] via 1.1.1.1, 00:00:24, Ethernet0/0
R2#
You can see that class A routes are coming as /8 and class B routes are coming as /16.
even inside "router eigrp 10" if you write network 10 as 10.1.1.0 it'll become 10.0.0.0 while doing show run.
no auto summary:
===============
R1(1.1.1.1)---------------------(1.1.1.2)R2
Now if i've configured eigrp with "no auto-summary" option
R1#sh running-config | sec eigrp
router eigrp 10
network 1.0.0.0
network 10.0.0.0
network 60.0.0.0 --------->this is my loopback ip address.
network 200.1.1.0
no auto-summary
R1#
R2#sh running-config | sec eigrp
router eigrp 10
network 1.0.0.0
network 20.0.0.0
network 130.1.0.0
no auto-summary
R2#
Now if suppose on R1 the router is having loopback as
"ip address 60.1.1.1 255.255.0.0"
the R2 will receive the route as /16 see below
R2#sh ip route eigrp
10.0.0.0/24 is subnetted, 1 subnets
D 10.1.1.0 [90/409600] via 1.1.1.1, 1d07h, Ethernet0/0
60.0.0.0/16 is subnetted, 1 subnets
D 60.1.0.0 [90/409600] via 1.1.1.1, 00:07:00, Ethernet0/0
D 200.1.0.0/16 [90/409600] via 1.1.1.1, 1d03h, Ethernet0/0
R2#
same way if the loopback ip address will be /24
the router R2 will receive the route with .
R2#sh ip route eigrp
10.0.0.0/24 is subnetted, 1 subnets
D 10.1.1.0 [90/409600] via 1.1.1.1, 1d07h, Ethernet0/0
60.0.0.0/24 is subnetted, 1 subnets
D 60.1.1.0 [90/409600] via 1.1.1.1, 00:00:09, Ethernet0/0
D 200.1.0.0/16 [90/409600] via 1.1.1.1, 1d03h, Ethernet0/0
R2 #
