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I am learning about route summarization and how it represent different subnets with a single address and single subnet mask,

for example,

if i have two subnets,

10.1.1.0 
10.1.2.0 

The summarized subnet with subnet mask is,

10.1.0.0/22

This is because, the two left octets in both networks are same while 6 bits in both the networks are same as well, so,

8 + 8 + 6 = 22

Now, if i try to implement this in Packet tracer,

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and enter show ip route command on the Router, then it gives the following output,

Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 2 subnets
C       10.1.1.0 is directly connected, FastEthernet0/0
C       10.1.2.0 is directly connected, FastEthernet0/1

Now, in the third last line it says,

     10.0.0.0/24 is subnetted, 2 subnets

My question is, why the summarized address is 10.0.0.0/24 instead of 10.1.0.0/22. ?

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It's because they are in the 10.0.0.0 network, and they are subnetted as /24. You are not seeing a route summary. If you had subnetted one differently than the other, say /25, you would get something like:

     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       10.1.1.0/25 is directly connected, FastEthernet0/0
C       10.1.2.0/24 is directly connected, FastEthernet0/1

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