Variable Length Subnet Masks

1
Variable Length Subnet Masks

• Luis Trejo
• 1a Reunión de Educación Continua
• CATC ITESM CEM
• Septiembre 2002

2
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.

3
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.

4
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).

5
Internet Scaling Problems

• Alternatives
• IPv6
• Subnetting
• VLSM
• CDIR
• NAT

6
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.
• 192.168.64.0/18
• RIP2, EIGRP, OSPF, BGP IS-IS.

7
Subnetting

• 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.

8
Subnetting

• 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.

9
Subnetting reduces the routing requirements of
the Internet
Private Network
130.5.32.0 130.5.64.0 130.5.96.0 130.5.128.0
130.5.160.0 130.5.192.0 130.5.224.0
130.5.0.0
10
Subnetting

• Benefits
• Route flapping (i.e. the rapid changes of routes)
  within the private network does not affect the
  Internet routing tables.

11
Subnetting

• 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.

12
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.

13
VLSM

• Benefits
• Efficient use of the organization s assigned IP
  address space.
• Route aggregation.

14
VLSM. Efficient use of the organization s
assigned IP address space

• Assume that a network administrator has decided
  to configure the 130.5.0.0/16 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!

15
VLSM. Efficient use of the organization s
assigned IP address space

• Assume in previous example that administrator is
  also allowed to configure the 130.5.0.0/16
  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.

16
VLSM. Route aggregation

• VLSM allows the recursive division of an
  organizations address space.
• It can be aggregated to reduce the amount of
  routing information at the top level.

17
VLSM permits route aggregation Reducing routing
table size
11.1.1.0/24 11.1.2.0/24 ... 11.1.252.0/24 11.1.254
.0/24
11.2.0.0/16 11.3.0.0/16 ... 11.252.0.0/16 11.254.0
.0/16
11.1.0.0/16
Router A
Router B
11.0.0.0/8
11.1.253.0/24
11.253.0.0/16
Router D
Router C
11.1.253.32/27 11.1.253.64/27 11.1.253.96/27 11.1.
253.128/27 11.1.253.160/27 11.1.253.192/27
11.253.32.0/19 11.253.64.0/19 ... 11.253.160.0/19
11.253.192.0/19
18
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.

19
VLSM permits the recursive division of a netrwork
prefix
11.1.1.0/24
11.1.2.0/24
11.1.0.0/16
11.1.253.32/27
11.2.0.0/16
11.1.253.64/27
11.1.253.0/24
11.3.0.0/16
11.1.254.0/24
11.1.253.160/27
11.0.0.0/8
11.253.32.0/19
11.1.253.192/27
11.252.0.0/16
11.253.64.0/19
11.253.0.0/16
11.254.0.0/16
11.253.160.0/19
11.253.192.0/19
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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.

21
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 levels
  largest subnet today?
• 4) How many hosts will there be on this levels
  largest subnet in the future?

22
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.

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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.

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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.

25
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.

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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.

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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.

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Requierments for VLSM Deployment

• Example
• If a packet destination IP address is 11.1.2.5
  and there are 3 network prefixes in the routing
  table (11.1.2.0/24, 11.1.0.0/16, and 11.0.0.0/8),
  the router would select the route to 11.1.2.0/24
  because it has the longest match with the
  destination IP address.

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Requierments for VLSM Deployment

• Destination 11.1.2.5 00001011.0000001.00000010.0
  0000101
• Route 1 11.1.2.0/24 00001011.0000001.00000010
  .00000000
• Route 2 11.1.0.0/16 00001011.0000001.00000000
  .00000000
• Route 3 11.0.0.0/8 00001011.0000000.00000000.
  00000000
• Best match is with the route having the longest
  prefix (most specific)

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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.

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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.

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VLSM example and exercises