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

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).

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.

Addressing a Network with Standard Subnetting Site A has two Ethernet networks Site B had one Ethernet network Site C had one Ethernet network 207.21.24.0 /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

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.

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). 207.21.24.0 Site A Site B Site C 25 users 10 users 8 users

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

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 255.255.255.128. 128 127 128 addresses 255

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 255.255.255.192. 128 127 64 addresses 64 192 63 191 255

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 255.255.255.224. 128 127 32 addresses 31 32 32 addresses 159 160 64 192 63 191 32 addresses 95 96 32 addresses 223 224 255

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 255.255.255.240. 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

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.

Addressing a Network Using VLSM To begin subnetting this network using VLSM, identify the LAN with the largest number of hosts. Subnet the address 207.21.24.0 /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

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

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 207.21.24.192 /30 207.21.24.196 /30 207.21.24.32 /27 207.21.24.64 /27 207.21.24.160 /28 207.21.24.176 /28

Addressing a Network Using VLSM Exercise 1 Your company has been assigned IP network 195.39.71.0 /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

Given the IP address 195.39.71.0 /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

Playing it safe, we will not use the first subnet (subnet 0). 64 128 192 We will start addressing with 195.39.71.64 /26. Headquarters needs 60 hosts, so we will assign them .64 - .127. Headquarters 60 hosts 26 bit mask or /26 (255.255.255.192)

The 5 Branch offices only need 12 hosts each. 64 128 192 160 Branch 1 12 hosts /28 (255.255.255.240) Branch 2 12 hosts /28 (255.255.255.240) Branch 3 12 hosts /28 (255.255.255.240) Branch 4 12 hosts /28 (255.255.255.240) The next address block available is the .128 - .191 block (64 addresses). Here we will apply VLSM. 144 176 Headquarters 60 hosts 26 bit mask or /26 (255.255.255.192) Using a /28 mask will give us 16 hosts at each location. This will take care of 4 of the Branch offices.

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 .192 - .255 block using a /28 mask. 64 128 192 160 Branch 1 12 hosts /28 (255.255.255.240) Branch 2 12 hosts /28 (255.255.255.240) Branch 3 12 hosts /28 (255.255.255.240) Branch 4 12 hosts /28 (255.255.255.240) 144 176 224 Branch 5 12 hosts /28 (255.255.255.240) Headquarters 60 hosts 26 bit mask or /26 (255.255.255.192) 208 240

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 (255.255.255.240) Branch 3 12 hosts /28 (255.255.255.240) 144 176 Branch 2 12 hosts /28 (255.255.255.240) Branch 4 12 hosts /28 (255.255.255.240) 224 232 WAN 5 Branch 5 12 hosts /28 (255.255.255.240) Here we will use a /30 mask to further subnet the subnets. Headquarters 60 hosts 26 bit mask or /26 (255.255.255.192) 228 236 208 240 216 WAN 1 WAN 2 WAN 3 WAN 4 212 220

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 (255.255.255.240) Branch 3 12 hosts /28 (255.255.255.240) 144 176 Branch 2 12 hosts /28 (255.255.255.240) Branch 4 12 hosts /28 (255.255.255.240) 224 232 WAN 5 Branch 5 12 hosts /28 (255.255.255.240) Headquarters 60 hosts 26 bit mask or /26 (255.255.255.192) 228 236 208 240 216 WAN 1 WAN 2 WAN 3 WAN 4 212 220

Address provided by ISP Applying the Addresses to the Topology Address provided by ISP 195.39.71.64 /26 195.39.71.128 /28 195.39.71.144 /28 195.39.71.160 /28 195.39.71.176 /28 195.39.71.192 /28 195.39.71.208 /30 195.39.71.212 /30 195.39.71.216 /30 195.39.71.220 /30 195.39.71.224 /30