adam's notes

Variable Length Subnet Mask (VLSM)

  • IPv4 address space is close to being exhausted
    • makes it difficult for ISPs to allocate public addresses
    • to mitigate this, supernetting is used
      • simplifies the information Internet routers need to locate IP networks
    • variable length subnet mask (VLSM) allows a network designer to allocate ranges of IP address to subnets that match the predicted need for numbers of subnets and hosts per subnet more closely
      • compliments supernetting
    • without VLSM, have to allocate subnetted ranges of addresses that are the same size and use the same subnet mask throughout the network
      • means that
        • some subnets have many wasted IP addresses
        • or additional routing interfaces must be installed to connect several smaller subnets together within a single building or department
    • VLSM allows different length netmasks to be used within the same IP network
      • allows more flexibility in the design process

Example

Consider a company with three sites

  • each with differing network sizes and IP address requirements
  • have subnets representing point-to-point WAN link between the routers

  • VLSM design proceeds by:
    • identifying the subnets with the most hosts
    • organizing the scheme in descending order
  • remember that each power of 2 is double the previous one:
48163264128256
  1. largest requirement is for 80 hosts
  • = 64 max values
  • not enough so next nearest match is
  • need 7 bits for host addressing
  • allows for 126 host addresses ()
  • prefix = /25 ()
  1. next requirement is met by a 5-bit host address space
  • allows for exactly 30 addresses, no room for growth
  • using 6 bits is safer, but we will use the closest match instead (/27)
  1. next three requirements are for 8, 12, and 12 hosts
  • all require 4 bits
    • gives 14 usable addresses
  1. routers use point-to-point links, so no more than two addresses will be required
  • can be met by selecting a /30 prefix

Final VLSM design:

|Office/Subnet|Required Number of IP Addresses|Mask Bits|Actual Number of IP Addresses|Prefix|
|---|---|---|---|---|
|Main Office 1 (Router A)|80|7|126|/25|
|Main Office 2 (Router A)|30|5|30|/27|
|Main Office 3 (Router A)|8|4|14|/28|
|Branch Office (Router B)|12|4|14|/28|
|Branch Office (Router C)|12|4|14|/28|
|Router A – Router B|2|2|2|/30|
|Router A – Router C|2|2|2|/30|
|Router B – Router C|2|2|2|/30|

  • all subnets except for Main Office 2 have room for growth
  • analyzing the final design, we find that there are 36 unused addresses at the end of the range
    • so would have been space to use a /26 prefix for the group of 30 hosts

IP address ranges generated by the VLSM design:

|Office|Subnet|Subnet Mask|Useable Host Address Range|Broadcast Address|
|---|---|---|---|---|
|Main Office 1 (Router A)|198.51.100.0/25|255.255.255.128|1—126|127|
|Main Office 2 (Router A)|198.51.100.128/27|255.255.255.224|129—158|159|
|Main Office 3 (Router A)|198.51.100.160/28|255.255.255.240|161—174|175|
|Branch Office (Router B)|198.51.100.176/28|255.255.255.240|177—190|191|
|Branch Office (Router C)|198.51.100.192/28|255.255.255.240|193—206|207|
|Router A – Router B|198.51.100.208/30|255.255.255.252|209—210|211|
|Router A – Router C|198.51.100.212/30|255.255.255.252|213—214|215|
|Router B – Router C|198.51.100.216/30|255.255.255.252|217—218|219|

VLSM network topology summarized below:

Graph View

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