IP EXAMPLES

Your company has a network ID of 212.107.8.0 .One router divides the network into four subnets. Each of the subnets will have a maximum of 50 hosts. What subnet mask should you use
255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
To subdivide this network address into four subnets, use the mask 255.255.255.192. Note that
255.255.255.192=11111111.111111111.11111111.11000000
the fourth octet is used to subdivide this class c network.The two ones in the last octet present the 192, which gives 22=4 subnets. The six zeros give 26-2=62 host IDs per subnet
A computer with the IP address 12.14.231.78 is located on subnet 1. Its subnet mask is 255.255.255.0. Which computers are not local to this computer ?
121.14.231.29
120.14.231.8
121.14.102.100
121.14.231.150
121.10.231.42
Computer2 is local to computer 1 if they have the same network ID. If the computer2 has a network id different than computer1’s ID then computer2 is remote. In other words, local hosts are on the same subnet, a remote host is on the different subnet
Applying the subnet mask 255.255.255.0 to the IP address 121.14.231.78 to obtain the network ID 12.14.231.0. Note that this subnet mask identifies the first three octects of the IP address as the network ID. Therefore, any IP address that starts with 12.14.231 is on the same subnet as the computer in the question. IP addresses that start with anything else are not local to the computer in question
Which are the legitimate subnet mask ?
255.250.0.0
255.126.0.0
255.255.248.0
255.255.248.0
255.254.128.0
255.240.192.0
255.128.0.0
255.255.255.224
subnet mask must be composed of contiguous 1s followed by contiguous 0s

Consider the IP address 128.10.150.1/19 and 128.10.158.8/19 . Are the computers local or remote in location to each other ?
To determine whether two computers are local or remote to each other, calculate the network ID for each computer.If the network ID are the same, the computers are local. In others words they are the same subnet. If the network id are different, the computers are remote. A router is required to send messages between the two computers.
Computer 1
IP Address 128.10.150.1/19 10000000 00001010 10010110 00000001
Subnet mask 11111111 11111111 11100000 00000000
Network ID 10000000 00001010 10000000 00000000
Network ID in CIDR notation 128.10.128.0/19
Computer2
IP Address 128.10.158.8/19 10000000 00001010 10011110 00001000
Subnet Mask 11111111 11111111 11100000 00000000
Network ID 10000000 00001010 10000000 00000000
Network ID in CIDR notation 128.10.128.0/19
Your company uses the Class C address 201.20.5.0. You need to create five subnets with up to 25 host IDs per subnet. Which subnet mask should you use for this type of network
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252
To subdivide this network address into five subnets, using the mask 255.255.255.224. Note that
255.255.255.224= 11111111.11111111.11111111.11100000
The fourth octect is used to subdivide the network. The subnet mask has three ones in the fourth octect. This gives 23 = 8 possible network (covering your 5). The five zeors in the fourth octect give 25 – 2 =30 possible hosts per subnet .( you need 25)
In CIDR notation this is 210.20.5.0/27