Subnetting (Part 2)

/17 - 2 subnets
/18 - 4 subnets
/19 - 8 subnets
/20 - 16 subnets
/21 - 32 subnets
/22 - 64 subnets
/23 - 128 subnets
/24 - 256 subnets
/25 - 512 subnets
/27 - 1024 subnets
/28 - 2048 subnets
/29 - 4096 subnets
/30 - 8192 subnets
/31 - 16384 subnets
/32 - 32768 subnets

Subtract 2 like (32768 - 2) because of the network and broadcast address

/17 - 32768 address
/18 - 16384 address
/19 - 8192 address
/20 - 4096 address
/21 - 2048 address
/22 - 1024 address
/23 - 512 address
/24 - 256 address
/25 - 128 address
/26 - 64 address
/27 - 32 address
/28 - 16 address
/29 - 8 address
/30 - 4 address
/31 - 2 address (point to point)
/32 - 0 address (only one device)

The picture below /21 should be 2046 not 2044

/23

172.30.0.0/16 

7 borrowed bits
2^7 = 128 subnets

# 9 host bits
2^9 - 2 = 510 usable hosts

172.30.0000 0000.0000 0000 = binary 
172.30.1111 1110.0000 0000 = 254 in decimal

This is /20
16171819 20 21 .22 23 24 25 26 27 28 29 30 31 32
00000000 00000000
01100000 00000000

which is 96 in decimal from binary

ORRRRRRR

172.21.111.201/20

172.21.0110 1111.0000 0000
172.21.0110 0000.0000 0000 = 172.21.96.0 in decimal form

JUST MINUS

LIKE

IF THE NUMBER IS 78

78 - 64 = 14

14 - 8 = 6

6 - 4 = 2

2 - 2 = 0

For the binary from decimal

/26 
192.168.91.78 
192.168.91.00000000
192.168.91.01001110 - binary of 78

192.168.91.01000000 - which is /26 which is 64

Then just add 
64 + 64 +64 +64 +64 +64

0 - network address
64 - network address

192.168.91.0 - network address
192.168.91.63 - broadcast address

192.168.91.64 - network address
102.168.91.127 - broadcast address (This is the answer)

ORRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR

192.168.91.78/26

192.168.0000 0000.0000 0000

172.16.0.0/16 

172.16.0.0/18 - 4 subnets

256 in a network which is the IPs
256 / 4 = 64 (4 is the number of subnets)

172.16.0.0 - network address
172.16.0.63 - broadcast address

172.16.0.64 - network address
172.16.0.127 - broadcast address

172.16.0.128 - network address
172.16.0.191 - broadcast address

172.16.0.192 - network address
172.16.0.255 - broadcast address

ORRRRRRRRR

172.16.0.0/16

Borrow 2 bits from the host portion of the IP which is 2^2 = 4 subnets

172.16.00 000000.00000000
172.16.01 000000.00000000 - binary form of 172.16.64.0 (network address 2nd subnet)
172.16.01 111111.11111111 - binary form of 172.16.64.255 (broadcast address 3rd sub)

64 subnets 

which is the /22 prefix