VLSM is similar to CIDR

Both recursively divide networks into small sub networks



The recursion is performed on the address space previously

assigned to an VLSM vs. CIDR

organization and is invisible to the global Internet.


CIDR permits the recursive allocation of an address blocked by an

Internet Registry to a high-level ISP, to a mid level to a lower level

ISP and finally to a private organization’s network.


The issues to be considered while

designing a network

1) How many total subnets does the organization need today?

2) How many total subnets will the organization need in the future?

3) How many hosts are there on the organization’s largest subnet today?

4) How many hosts will there be on the organization’s largest subnet in

the future?

Solution to Problem 1:

To support 26 hosts on a subnet, a minimum of 5 bits is needed in the host portion of the

address. 5 bits result in 30 possible host addresses (2^5-2). The other 3 bits in the last

octet can be added to the default 24-bit Class C mask. Thus, a 27-bit mask can be used to

create the following subnets:-

Subnet # Subnet Address

0 /27

1 /27


2 /27

3 /27

4 /27

5 /27

6 /27

Sub-subnet 0 /30

Sub-subnet 1 /30

Sub-subnet 2 /30

Sub-subnet 3 /30

Sub-subnet 4 /30

Sub-subnet 5 /30

Sub-subnet 6 /30


To maximize the address space, the /27 subnet is further subnetted using a

30-bit mask. This creates subnets that can be used on point-to-point links with minimal

waste, because each subnet contains only 2 possible host addresses.

R1  -R  -B


R2 -R -B

R3 -R -B

R4 -R -B


R2-    R4-

Ip rote

R1 iprote



R4 iprote