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Variable Length Subnet Masks. Luis Trejo 1a Reunión de Educación Continua CATC ITESM CEM Septiembre 2002. Internet Scaling Problems. Over the past years, the Internet has experienced 2 major scaling issues as it has struggled to provide continous and interrupted growth

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variable length subnet masks

Variable Length Subnet Masks

Luis Trejo

1a Reunión de Educación Continua


Septiembre 2002

internet scaling problems
Internet Scaling Problems
  • Over the past years, the Internet has experienced 2 major scaling issues as it has struggled to provide continous and interrupted growth
    • The eventual exhaustion of the IPv4 address space.
    • The ability to route traffic between the ever increasing number of networks that compromse the Internet.
internet scaling problems1
Internet Scaling Problems
  • IPv4 defines a 32-bit address.
  • 232 (4,294,967,296) adresses available.
  • The address shortage problem is aggravated by the fact that portions of the IP address space have not been efficiently allocated.
  • IP was first standarized in September 1981.
  • 5 classes: A, B, C, D and E.
internet scaling problems2
Internet Scaling Problems

Disign problem:

  • Class C networks are too small (254 hosts).
  • Next option is class B, which is too big (65,534 hosts).
internet scaling problems3
Internet Scaling Problems
  • Alternatives:
    • IPv6
    • Subnetting
    • VLSM
    • CDIR
    • NAT
classful vs classless addressing
Classful vs Classless Addressing
  • Classful:
    • Size defined by the class (A, B, C, D, E).
    • Fixed network portion.
    • RIP & IGRP are classful routing protocols.
  • Classless:
    • Network portion can be any size.
    • Protocol sends subnetting (prefix) information with routes.
  • In 1985, RFC 950 defined a standard procedure to support subnetting, or division, of a single class A, B, or C network number into smaller pieces.
  • Subnetting was introduced to overcome some of the following problems Internet was experiencing:
    • Internet routing tables started to grow
    • Local administrators had to request another network number from the Internet before a new network could be installed at their site.
  • Benefits:
    • The size of the global Internet routing table does not grow because the site administrator does not need to obtain additional adress space and the routing advertisments for all of the subnets are combined into a single routing table entry.
    • The local administrator has the flexibility to deploy additional subnets without obtaining a new network form the Internet.


Subnetting reduces the routing requirements of the Internet

Private Network

  • Benefits:
    • Route flapping (i.e. the rapid changes of routes) within the private network does not affect the Internet routing tables.
  • Drawbacks
    • Once the desinged has been established, it remains static. It locks the organization into a fixed-number of fixed-sized subnets.
    • A lot IP addresses are wasted for subnets with small number of hosts.
variable length subnet masks vlsm
Variable Length Subnet Masks (VLSM)
  • In 1987, RFC 1009 specified that a subnetted network could use more than one subnet mask.
  • When an IP network is assigned more than one subnet mask, it is considered a network with variable length subnet masks.
  • RIP-1 permits only a single subnet mask
    • It does not provide subnet mask information as part of its routing table update messages.
  • Benefits
    • Efficient use of the organization’ s assigned IP address space.
    • Route aggregation.
vlsm efficient use of the organization s assigned ip address space
VLSM. Efficient use of the organization’ s assigned IP address space
  • Assume that a network administrator has decided to configure the network with a /22 extended-network prefix.
  • This disign allows for 64 subnets with 1,022 hosts each.
  • Fine if the organization plans to deploy a number of large subnets.
  • What about the occasional small subnet containing only 20 or 30 hosts?
  • About 1,000 IP host addresses wasted for every small occasional subnet!
vlsm efficient use of the organization s assigned ip address space1
VLSM. Efficient use of the organization’ s assigned IP address space
  • Assume in previous example that administrator is also allowed to configure the network with a /26 extended-network-prefix.
  • /26 permits 1024 subnets with 62 hosts each.
  • The /26 prefix would be ideal for small subnets with less than 60 hosts, while /22 prefix is well suited for larger subnets up to 1000 hosts.
vlsm route aggregation
VLSM. Route aggregation
  • VLSM allows the recursive division of an organization´s address space.
  • It can be aggregated to reduce the amount of routing information at the top level.


VLSM permits route aggregation

Reducing routing table size



Router A

Router B

Router D

Router C


vlsm operation
VLSM operation
  • Conceptually, a network is divided into subnets, some of the subnets are further divided into sub-subnets, and some of the sub-subnets are divided into sub2-subnets.

VLSM permits the recursive division of a netrwork prefix

vlsm operation1
VLSM operation
  • The recursive process does not require the same extended-network-prefix be assigned at each level of recursion.
  • The recursive subdivision can be carried out as far as the network administrator needs to take it.
vlsm design considerations
VLSM Design Considerations

At each level of the hierarchy:

  • 1) How many total subnets does this level need today?
  • 2) How many total subnets does this level need in the future?
  • 3) How many hosts are there on this level´s largest subnet today?
  • 4) How many hosts will there be on this level´s largest subnet in the future?
vlsm design considerations example
VLSM Design Considerations (example)
  • Assume a network is spread out over a number of sites.
  • An organization has 3 campuses today.
  • It will need 3 bits of subnetting to allow growth (8 subnets).
  • Within each campus a second level of subnetting will identify a building.
  • Within each building a third level of subnetting will identify an individual workgroup.
vlsm design considerations example1
VLSM Design Considerations (example)
  • From this hierarchical model, the top level is determined by the number of campuses.
  • The mid-level by the number of buildings at each site.
  • The lowest level by the number of workgroups.
vlsm design considerations example2
VLSM Design Considerations (example)
  • The deployment of a hierarchical subnetting scheme requires careful planning.
  • At the bottom level, the designer must be sure that the leaf subnets are large enough to support the required number of hosts.
  • The addresses from each site will be aggregable into a single address block that keeps the backbone routing tables from becoming too large.
requierments for vlsm deployment
Requierments for VLSM Deployment
  • Three prerequisites:
    • The routing protocols must carry extended-network-prefix information with each routing update.
    • All routers must implement a consistent forwarding algorithm based on the longest match.
    • For route aggregation to occur, addresses must be assigned so that they have topological significance.
requierments for vlsm deployment1
Requierments for VLSM Deployment

Routing protocols

  • OSPF, IS-IS, RIP-2, EIGRP allow the deployment of VLSM by providing the extended-network-prefix length or mask value along with each route advertisement.
  • This permits each subnetwork to be advertised with its corresponding prefix length or mask.
requierments for vlsm deployment2
Requierments for VLSM Deployment

Forwarding algorithm based on longest match

  • A route with a longer e-n-p describes a smaller set of destinations than the same route with a shorter e-n-p.
  • Then, a route with a longer e-n-p is said to be “more specific”.
  • A route with a shorter e-n-p is said to be “less specific”.
  • Routers must use the route with the longest matching e-n-p (most specific matching route) when forwarding traffic.
requierments for vlsm deployment3
Requierments for VLSM Deployment


  • If a packet destination IP address is and there are 3 network prefixes in the routing table (,, and, the router would select the route to because it has the longest match with the destination IP address.
requierments for vlsm deployment4
Requierments for VLSM Deployment

Destination = 00001011.0000001.00000010.00000101

* Route #1 = 00001011.0000001.00000010.00000000

Route #2 = 00001011.0000001.00000000.00000000

Route #3 = 00001011.0000000.00000000.00000000

Best match is with the route having the longest prefix (most specific)

requierments for vlsm deployment5
Requierments for VLSM Deployment

Topological significant address assignment

  • Hierarchical routing requires that addresses be assigned to reflect the actual network topology.
  • Routing information is reduced by taking the set of addresses assigned to a particular region of the topology, and aggregating them into a single routing update for the entire set.
  • This can be done recursively at various points within the hierarchy of the routing topology.
requierments for vlsm deployment6
Requierments for VLSM Deployment

Topological significant address assignment

  • If addresses do not have a topological significance, aggregation cannot be performed and the size of routing tables would not be reduced.