When you're subnetting you're borrowing bits from the host ID to create a subnet ID. In the example you're provided, it's a /24, so the first 24 bits are network ID and the last 8 are host ID.

Each octet has 8 bits, so in binary that IP address looks like this:

/------------Network ID------------\ /-Host ID-\ 11000000 . 10101000 . 11011100 . 00000000 - network address 192 . 168 . 220 . 0 11111111 . 11111111 . 11111111 . 00000000 - Subnet 255 . 255 . 255 . 0

If you want to split this network into two networks, you need to borrow a bit from the host ID, so your networks will now look like this:

/------------Network ID------------\ /Host ID\ 11000000 . 10101000 . 11011100 . 0 0000000 - first network 192 . 168 . 220 . 0 11111111 . 11111111 . 11111111 . 10000000 - Subnet 255 . 255 . 255 . 128

/------------Network ID------------\ /Host ID\ 11000000 . 10101000 . 11011100 . 1 0000000 - network address 192 . 168 . 220 . 128 11111111 . 11111111 . 11111111 . 10000000 - Subnet 255 . 255 . 255 . 128

So if you borrow 1 bit from the host ID, you're effectively saying that you can have two subnets (where the Subnet ID bit is a 0 or a 1), and that each subnet can contain as many hosts as equal to the remaining number of bits (127) -1 (the broadcast ID).