Consider the division of a class-C network into subnets of size 70, 40, 25, and 20, i.e. four (4) distinct subnets must be provided able to accommodate the hosts.

Assume that the sites IPv4 network address is 197.5.5.0 /24

(mask 255.255.255.0)

Can this even be done?

A class C network only has 28= 256 host addresses (actually only 254 discounting for all 0s and all 1s)

	Subnet mask	#subnets (host part in yellow)		#hosts per subnet		IP range for each subnet
A	197.5.5.0 / 25 255.255.255.128 128=10000000	(2) 0 | 0000000 1 | 0000000		128 128		197.5.5.0-197.5.5.127 197.5.5.128-197.5.5.255
B	197.5.5.0 / 26 255.255.255.192 192= 11000000	(4) 00 | 000000 01 | 000000 10 | 000000 11 | 000000		64 64 64 64		197.5.5.0-197.5.5.63 197.5.5.64-197.5.5.127 197.5.5.128-197.5.5.191 197.5.5.192-197.5.5.255
C	197.5.5.0 / 27 255.255.255.224 224=1110 0000	(8) 000 | 00000 001 | 00000 010 | 00000 011 | 00000 etc.		32 32 32 32		197.5.5.0-197.5.5.31 197.5.5.32-197.5.5.63 197.5.5.64-197.5.5.95 197.5.5.96-197.5.5.127

This seems to do the trick. Judicious subnetting provides:

A: 2 subnets with 128 host addresses each

B: 4 subnets with 64 host addresses each

C: 8 subnets with 32 host addresses each

Original task was to break down the class C network into subnet to provide homes for 70, 40, 25, 20 machines on separate networks.

70 machines can be assigned to one of the 2 subnets with 128 addresses

40 machines can be assigned to one of the 4 subnets with 64 addresses

25 machines can be assigned to one of the 8 subnets with 32 addresses

20 machines can be assigned to a different subnet with 32 addresses

Does it matter which of the respective subnets are chosen?

Assign the 70 hosts to this subnet:

197.5.5.0 197.5.5.127 (subnet mask 255.255.255.128) (/25)

Assign the 40 hosts to this subnet:

197.5.5.0 197.5.5.63 (subnet mask 255.255.255.192) (/26)

Anything noteworthy?

Does it matter?

Take a random address from each of the two subnets,

197.5.5.36 / 25 11000101.00000101.00000101.0|0100100

apply subnet mask 255.255.255.128: 11111111.11111111.11111111.1|0000000

Result (this is what the router uses to make its routing decision): 11000101.00000101.00000101.00000000

197.5.5.12 / 26 11000101.00000101.00000101.00|001100

apply subnet mask 255.255.255.192 11111111.11111111.11111111.11|000000

Result: 11000101.00000101.00000101.00 000000

These are the steps a router takes to determine how to route to the right subnet. However, the results are in both cases the same (even though the subnet masks are different).

The router is now confused and not able to distinguish between the two subnets.

The router does NOT take the length of the network address into account.

So, you cannot take any random subnet from A, B, or C.

You have to pick subnets from A, B, and C so that their ranges dont overlap.

Example:

From A take: 197.5.5.128-197.5.5.255

From B take: 197.5.5.64-197.5.5.127

From C take: 197.5.5.0-197.5.5.31

197.5.5.32-197.5.5.63

the question is;

This is not the only possible selection of subnets that do not overlap.

Find another selection of four subnets from A,B,C that do NOT overlap (and still solve the original task of accommodation 70,40,25, and 20 devices).