Subnet Mask

Subnet Mask

For Host, assign 0 from right side.
For Subnet (Network), assign 1 from left side.
Rule: First calculate network bits.
Assign network bits from left to right.
Assign host bits rest ot the field(octet).


Example 1:
You are designing a subnet mask for the 10.0.0.0/8 network. You want 200 subnets/networks. What subnet mask should you use?
Example 2:
You have a Class B network and need 28 subnets. What is your mask?
Example 3:
You need to have a Class B network address subnetted into exactly 510 subnets. What subnet mask would you assign?
Example: 4:
You are designing a subnet mask for the 172.16.0.0/16 network. You want 100 subnets/networks. What subnet mask should you use?
Example 5:
You are designing a subnet mask for the 172.16.0.0/16 network. You want 500 subnets/networks. What subnet mask should you use?
Example 6:
You have a network ID of 192.168.0.0 and need to divide it into 6 subnets. What subnet mask will you use for largest possible number of hosts per subnet?
Example 7:
You are designing a subnet mask for the 192.168.10.0/24 network. You want 5 subnets/networks. What subnet mask should you use?
Example 8:
You are designing a subnet mask for the 192.168.10.0/24 network. You want 10 subnets/networks. What subnet mask should you use?

Class A

Example 1:

You are designing a subnet mask for the 10.0.0.0/8 network. You want 200 subnets/networks. What subnet mask should you use?

Solution:

Class A addresses use a default mask of 255.0.0.0
200 networks would need 8 bits of subnet.
Subnet mask = 255. 00000000. 00000000. 00000000
For 200 subnets, assign eight 1s from left side.
New subnet mask = 255. 11111111. 00000000. 00000000
                = 255. 255. 0. 0
Subnet bits = 8 or 8 bits available for subnet addresses.
Subnets = 2^8 = 256.
Number of subnets possible = 256.
The best mask for this network = 255. 255. 0. 0
    

Class B

Example 2:

You have a Class B network and need 28 subnets. What is your mask?

Solution:

Class B addresses use a default mask of 255.255.0.0
For 28 subnets, we have to assign 5 bits to third octet or borrow
5 bits from third octet.
Subnet mask = 255. 255. 00000000. 00000000
For 28 subnets, assign five 1s from left side.
New subnet mask = 255. 255. 11111000. 00000000
                = 255. 255. 248. 0
Subnet bits = 5 or 5 bits available for subnet addresses.
Subnets = 2^5 = 32.
Number of subnets possible = 32.
The best mask for this network = 255. 255. 248. 0
    

Example 3:

You need to have a Class B network address subnetted into exactly 510 subnets. What subnet mask would you assign?

Solution:

Class B addresses use a default mask of 255.255.0.0
For 510 subnets, we have to assign 9 bits to third
and forth octet or borrow 9 bits from third and forth octet.
Subnet mask = 255. 255. 00000000. 00000000
For 510 subnets, assign nine 1s from left side.
New subnet mask = 255. 255. 11111111. 10000000
                = 255. 255. 255. 128
Subnet bits = 9 or 9 bits available for subnet addresses.
Subnets = 2^9  = 512.
For 512 subnets, we would assign 255. 255. 255. 128 subnet mask.
    

Example 4:

You are designing a subnet mask for the 172.16.0.0/16 network. You want 100 subnets/networks. What subnet mask should you use?

Solution:

Class B addresses use a default mask of 255.255.0.0
100 networks would need  7 bits of subnet.
Subnet mask = 255. 255. 00000000. 00000000
For 100 subnets, assign seven 1s from left side.
New subnet mask = 255. 255. 11111110. 00000000
                = 255. 255. 254. 0
Subnet bits = 7 or 7 bits available for subnet addresses.
Subnets = 2^7 = 128.
The best mask for this network = 255. 255. 254. 0
    

Example 5:

You are designing a subnet mask for the 172.16.0.0/16 network. You want 500 subnets/networks. What subnet mask should you use?

Solution:

Class B addresses use a default mask of 255.255.0.0
  500 networks would need  9 bits of subnet.
  Subnet mask = 255. 255. 00000000. 00000000
  For 500 subnets, assign nine 1s from left side.
  New subnet mask = 255. 255. 11111111. 10000000
                  = 255. 255. 255. 128
  Subnet bits = 9 or 9 bits available for subnet addresses.
  Subnets = 2^9 = 512.
  The best mask for this network = 255. 255. 255. 128
    

Class C

Example 6:

You have a network ID of 192.168.0.0 and need to divide it into 6 subnets. What subnet mask will you use for largest possible number of hosts per subnet?

Solution:

Class C addresses use a default mask of 255.255.255.0
6 subnets would need 3 bits of subnet.
Subnet mask = 255. 255. 255. 00000000
For 6 subnets, assign three 1s from left side.
New subnet mask = 255. 255. 255. 11100000
                = 255. 255. 255. 224
Subnet bits = 3 or 3 bits available for subnet addresses.
Subnets = 2^3  = 8.
For 6 subnets and for largest possible number of hosts per subnet,
we would assign 255. 255. 255. 224 subnet mask.
The best mask for this network = 255. 255. 255. 224
    

Example 7:

You are designing a subnet mask for the 192.168.10.0/24 network. You want 5 subnets/networks. What subnet mask should you use?

Solution:

Class C addresses use a default mask of 255.255.255.0
5 subnets would need 3 bits of subnet.
Subnet mask = 255. 255. 255. 00000000
For 5 subnets, assign three 1s from left side.
New subnet mask = 255. 255. 255. 11100000
                = 255. 255. 255. 224
Subnet bits = 3 or 3 bits available for subnet addresses.
Subnets = 2^3 = 8.
For 5 subnets, we would assign 255. 255. 255. 224 subnet mask.
    

Example 8:

You are designing a subnet mask for the 192.168.10.0/24 network. You want 10 subnets/networks. What subnet mask should you use?

Solution:

Class C addresses use a default mask of 255.255.255.0
10 subnets would need 4 bits of subnet = 28 bits of network
(32 – 4 = 28)
Subnet mask = 255. 255. 255. 00000000
For 10 subnets, assign four 1s from left side.
New subnet mask = 255. 255. 255. 11110000
                = 255. 255. 255. 240
Subnet bits = 4 or 4 bits available for subnet addresses.
Subnets = 2^4 = 16.
For 10 subnets, we would assign 255. 255. 255. 240 subnet mask.