VLSM

Variable Length Subnet Mask

VARIABLE LENGTH SUBNET MASK

VLSM is the process of creating subnets of different sizes to make your use of network addresses more efficiently. More complicated than FLSM (Fixed length subnet masks) where all of the subnets use the same prefix length. As we know thats not the case in the real world where different subnet sizes are required (point-to-point, management, Data Centres, PoOP's etc)

As can be seen we will have to assign different prefix's for the different LAN's and P-t-P WAN

The easiest way is to:

  • Assign the largest subnet at the START of the address space, then the next largest until all subnets have been assigned.

    • Tokyo LAN A

    • Toronto LAN B

    • Toronto LAN A

    • Tokyo LAN B

    • Point-to-Point Connection

192.168.1.0/24

Tokyo LAN A

  • Step 1:

    • Which formula to use Networks or Hosts?

      • 110 Hosts -> (2^n-2)

      • Bits from right to left n=7 (128 -2 = 126 Hosts)

Therefore 2 networks /25 with a range of 128 and subnet mask 255.255.255.129. The hosts per subnet is 2^7-2= 126 Hosts

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.1.0/25 192.168.1.1 192.168.1.126 192.168.1.127 192.168.1.128/25 192.168.1.129 192.168.1.254 192.168.1.255

Toronto LAN B

We will now use the second network available which is 192.168.1.128/25.

  • Step 1:

    • Which formula to use Networks or Hosts?

      • 45 Hosts -> (2^n-2)

      • Bits from right to left n=6 (64 -2 = 62 Hosts)

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.1.128/26 192.168.1.129 192.168.1.190 192.168.1.191 192.168.1.192/26 192.168.1.193 192.168.1.254 192.168.1.255

Toronto LAN A

We will now use the second network available which is 192.168.1.192/26.

  • Step 1:

    • Which formula to use Networks or Hosts?

      • 29 Hosts -> (2^n-2)

      • Bits from right to left n=5 (32 -2 = 30 Hosts)

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.1.192/27 192.168.1.193 192.168.1.222 192.168.1.223 192.168.1.224/27 192.168.1.225 192.168.1.254 192.168.1.255

Tokyo LAN B

We will now use the second network available which is 192.168.1.224/27

  • Step 1:

    • Which formula to use Networks or Hosts?

      • 8 Hosts -> (2^n-2)

      • Bits from right to left n=4 (16 -2 = 14 Hosts)

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.1.224/28 192.168.1.225 192.168.1.238 192.168.1.239 192.168.1.240/28 192.168.1.241 192.168.1.254 192.168.1.255

Point - to - Point

We will now use the second network available which is 192.168.1.240/28

  • Step 1:

    • Which formula to use Networks or Hosts?

      • 2 Hosts -> (2^n-2)

      • Bits from right to left n=2 (4 -2 = 2 Hosts)

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.1.240/30 192.168.1.241 192.168.1.242 192.168.1.243 192.168.1.244/30 192.168.1.245 192.168.1.248/30 192.168.1.252/30 ------DONE

LAB

Step 1: Identify Networks

Subnet the 192.168.5.0/24 - Largest LAN to Smallest LAN

  • LAN2 - 64 Hosts

  • LAN1 - 45 Hosts

  • LAN3 - 14 Hosts

  • LAN4 - 9 Hosts

  • Point-to-Point Link

Step 2: Allocate VLSM to LAN's

  • LAN2 - 64 Hosts

    • 192.168.5.0/24

    • Hosts -> (2^n-2) = 128-2 = 126 Hosts

      • n= 7

      • On /32 - right to left go back 7 bits /25

      • Range of 128

      • 126 Hosts

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.5.0/25 192.168.5.1 192.168.5.126 192.168.5.127 192.168.5.128/25

  • LAN1 - 45 Hosts

    • 192.168.5.128/27 - Taken from the next available subnet from above

    • Hosts 45 -> (2^n-2) = 64-2 = 62 Hosts

      • n=6

      • On /32 - right to left go back 6 bits /26

      • Range of 64

      • 62 Hosts

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.5.128/26 192.168.5.129 192.168.5.190 192.168.5.191 192.168.5.192/26

  • LAN3 - 14 Hosts

    • 192.168.5.192/26 - Taken from the next available subnet from above

    • Hosts 14 -> (2^n-2) =

      • n=4 (on /32 right to left come back 4 bits) = /28

      • Range of 16

      • 14 hosts

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.5.192/28 192.168.5.193 192.168.5.206 192.168.5.207 192.168.5.208/28

  • LAN 4 - 9 Hosts

    • 192.168.5.208/28 - Taken from the next available subnet from above

    • Hosts 9 -> (2^n-2)

      • n=4 (will take the next available subnet in the /28

      • Range 16

      • 14 Hosts

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.5.208/28 192.168.5.209 192.168.5.222 192.168.5.223 192.168.5.224/28

  • Point-to-Point - 2 Hosts

    • 192.168.5.160/28

    • Hosts 2 -> (2^n-2)

      • n=2 (from /32 right to left move 2 bits) /30

      • Range 4

      • 2 Hosts

Network Address 1st Usable Address Last Usable Address Broadcast Address 192.168.5.224/30 192.168.5.225 192.168.5.226 192.168.5.227 192.168.5.228/30 192.168.5.229 192.168.5.230 192.168.5.231 192.168.5.232/30 ----etc

DONE !! :)

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