When you subnet a network into multiple subnets, the first subnet created is called the Subnet Zero whose network address will be exactly be the same as the actual Network Address.
This can create confusions and importantly in legacy hardware can cause routing issues. However, newer network routers and Cisco Routers running Cisco IOS 12.x can handle these Subnet-Zero without any problem. Cisco IOS 12.x by default enables subnet-zero.
Let's say we subnet a network 172.20.0.0/16 network and we borrow 1st 3 bits of the 3rd octet to create multiple subnets.
The 1st subnet will be
172.20.0.0/19
[1st 3 bits are set to "0"]
which is exactly the same as the actual network itself.
To enable Subnet-Zero
ciscorouter# conf term
ciscorouter(config)# ip subnet-zero
To disable Subnet-Zero if you think this can cause issues
ciscorouter(config)# no ip subnet-zero
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