1. Variable Length Subnet Mask (VLSM)
CCNA
Variable Length Subnet Mask
(VLSM)

2. What is VLSM?
A Variable Length Subnet Mask (VLSM) is a means of allocating IP addressing resources to subnets according to their individual need rather than some general network-wide rule.
VLSM allows an organization to use more than one subnet mask within the same network address space. It is often referred to as ‘subnetting a subnet’, and can be used to maximize addressing efficiency.
Large subnets are created for addressing LANs and small subnets are created for WAN links (a 30 bit mask is used to create subnets with only two host).

3. Subnetting vs. VLSM
Subnetting allows you to divide big networks into smaller, equal-sized slices.
VLSM allows you to divide big networks into smaller, different-sized slices. This enables you to make maximum use of your valuable IP address space.

4. Addressing a Network with Standard Subnetting
Site A has two Ethernet networks
Site B had one Ethernet network
Site C had one Ethernet network
/24
How many network addresses are needed?
How many hosts are needed for the largest LAN?
How many bits need to be borrowed to address this network?
Site A
Site B
Site C
25 users
10 users
8 users

5. Addressing a Network with Standard Subnetting
Site A
Site B
Site C
25 users
10 users
8 users
Site A has two Ethernet networks
Site B had one Ethernet network
Site C had one Ethernet network
If we borrow 3 bits from a class C address, that will give us eight networks. Each network will have 30 usable addresses.
It will take four network addresses to accommodate the Ethernet networks at each site. That leaves us with four extra networks.
There is also a point-to-point WAN connection between each site. These two connections will take up two of the remaining networks.

6. Addressing a Network with Standard Subnetting
Borrowing 3 bits will meet the current needs of the company, but it leaves little room for growth.
Each network will have 30 usable addresses, including the point-to-point WAN links (which only require two addresses).
Site A
Site B
Site C
25 users
10 users
8 users

7. Subnetting in a Box 256 addresses
To begin, in a class C network there are 256 addresses. When we subnet the address, we break it down in to smaller units or subnets.
256 addresses
255

8. Subnetting in a Box 128 addresses
If we were to borrow 1 bit, it would break the 256 addresses in to two parts (networks) with each part (subnet) having 128 addresses.
The subnet mask would be
128
127
128 addresses
255

9. Subnetting in a Box 64 addresses
If we were to borrow 2 bits, it would break each of these 2 networks in half again. This would give us 4 networks, each with 64 addresses.
The subnet mask would now be
128
127
64 addresses 64
192 63
191
255

10. Subnetting in a Box 32 addresses 32 addresses 32 addresses
If we were to borrow 3 bits, it would break each of these 4 networks in half again. This would give us 8 networks, each with 32 addresses.
The subnet mask would now be
128
127
32 addresses 31 32
32 addresses
159
160 64
192 63
191
32 addresses 95 96
32 addresses
223
224
255

11. Subnetting in a Box 16 addresses 16 addresses 16 addresses
If we were to borrow 4 bits, it would break each of these 8 networks in half again. This would give us 16 networks, each with 16 addresses.
The subnet mask would now be
128
127 32
160
16 addresses
16 addresses
16 addresses
16 addresses 16 15 48 47
144
143
176
175 64
192 63
191 31
159 96
224
16 addresses
16 addresses
16 addresses
16 addresses 80 79
112
111
208
207
240
239 95
223
255

12. Addressing a Network Using VLSM
When using VLSM to subnet an address, not all of the subnets have to be the same size.
A different subnet mask may be applied to some of the subnets to further subnet the address.
In order to take advantage of VLSM, the proper routing protocol must be selected.
Not all routing protocols share subnetting information in their routing table updates.

13. Addressing a Network Using VLSM
To begin subnetting this network using VLSM, identify the LAN with the largest number of hosts. Subnet the address /24 based on this information.
Site A has two Ethernet networks (25 hosts each)
Site B had one Ethernet network (10 hosts)
Site C had one Ethernet network (8 hosts)
Site A
Site B
Site C
25 users
10 users
8 users

14. Addressing a Network Using VLSM
Subnet 1 & 2 can be used to address Site A Ethernet networks. Subnet 5 can be subnetted to accommodate Site B & C Ethernet networks. Subnet 6 can be subnetted to accommodate the WAN links.
Site A
Site B
Site C
25 users
10 users
8 users
Site A
Site B
Site C
Free Addresses
Free Addresses
WAN 1 & 2
Site B & C
WAN links

15. Addressing a Network Using VLSM
Through applying VLSM, the topology was able to be addressed and still have two complete subnets available for future growth.
Site A
Site B
Site C
25 users
10 users
8 users
/30
/30
/27
/27
/28
/28

16. Addressing a Network Using VLSM Exercise 1
Your company has been assigned IP network /24. Given that headquarters (60 hosts) is connected to five branch offices (12 hosts each) by a WAN link, and to an ISP (the ISP owns the addresses on that link), determine an appropriate IP addressing scheme.
Headquarters
Branch 1
60 users
12 users
Branch 2
Branch 3
Branch 4
Branch 5
ISP

17. Given the IP address /24, subnet according to the largest subnet needed. (Headquarters 60 hosts)
128
127 64
192 63
191
You would need to borrow 2 bits or /26. This would give you 4 networks with 64 host addresses on each subnet.
255

18. Playing it safe, we will not use the first subnet (subnet 0).64
128
192
We will start addressing with /26. Headquarters needs 60 hosts, so we will assign them
Headquarters
60 hosts
26 bit mask or /26
( )

19. The 5 Branch offices only need 12 hosts each.64
128
192
160
Branch 1 12 hosts /28 ( )
Branch 2 12 hosts /28 ( )
Branch 3 12 hosts /28 ( )
Branch 4 12 hosts /28 ( )
The next address block available is the block (64 addresses). Here we will apply VLSM.
144
176
Headquarters
60 hosts
26 bit mask or /26
( )
Using a /28 mask will give us 16 hosts at each location. This will take care of 4 of the Branch offices.

20. To obtain a block for Branch 5, we will need to subnet the. 192 -
To obtain a block for Branch 5, we will need to subnet the block using a /28 mask. 64
128
192
160
Branch 1 12 hosts /28 ( )
Branch 2 12 hosts /28 ( )
Branch 3 12 hosts /28 ( )
Branch 4 12 hosts /28 ( )
144
176
224
Branch 5 12 hosts /28 ( )
Headquarters
60 hosts
26 bit mask or /26
( )
208
240

21. Here we will use a /30 mask to further subnet the subnets.
Now we need to address the 5 WAN links that connect to the Branch offices. These are point-to-point connections and only require 2 addresses. 64
128
192
160
Branch 1 12 hosts /28 ( )
Branch 3 12 hosts /28 ( )
144
176
Branch 2 12 hosts /28 ( )
Branch 4 12 hosts /28 ( )
224
232
WAN 5
Branch 5 12 hosts /28 ( )
Here we will use a /30 mask to further subnet the subnets.
Headquarters
60 hosts
26 bit mask or /26
( )
228
236
208
240
216
WAN 1
WAN 2
WAN 3
WAN 4
212
220

22. Subnet 0 could be used later if needed for future growth of HQ or for more Branch offices.64
128
192
160
Branch 1 12 hosts /28 ( )
Branch 3 12 hosts /28 ( )
144
176
Branch 2 12 hosts /28 ( )
Branch 4 12 hosts /28 ( )
224
232
WAN 5
Branch 5 12 hosts /28 ( )
Headquarters
60 hosts
26 bit mask or /26
( )
228
236
208
240
216
WAN 1
WAN 2
WAN 3
WAN 4
212
220

23. Address provided by ISP
Applying the Addresses to the Topology
Address provided by ISP
/26
/28
/28
/28
/28
/28
/30
/30
/30
/30
/30