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Variable Length Subnet Masks

Luis Trejo

1a Reunión de Educación Continua

CATC ITESM CEM

Septiembre 2002

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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.

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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.

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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).

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Internet Scaling Problems Alternatives:

IPv6 Subnetting VLSM CDIR NAT

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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.

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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.

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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.

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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

Internet

130.5.0.0

Subnetting reduces the routing requirements of the Internet Private Network

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Subnetting Benefits:

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

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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.

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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.

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VLSM

Benefits Efficient use of the organization’ s

assigned IP address space. Route aggregation.

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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!

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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.

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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.

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11.2.0.0/1611.3.0.0/16...11.252.0.0/1611.254.0.0/16

11.253.32.0/1911.253.64.0/19...11.253.160.0/1911.253.192.0/19

11.1.253.32/2711.1.253.64/2711.1.253.96/2711.1.253.128/2711.1.253.160/2711.1.253.192/27

11.1.1.0/2411.1.2.0/24...11.1.252.0/2411.1.254.0/24

Router C Router D

Router BRouter A

Internet11.253.0.0/16 11.1.253.0/24

11.1.0.0/16

11.0.0.0/8

VLSM permits route aggregationReducing routing table size

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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.

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11.0.0.0/8

11.1.0.0/16

11.252.0.0/16

11.3.0.0/16

11.2.0.0/16

11.253.0.0/16

11.254.0.0/16

11.253.32.0/19

11.253.64.0/19

11.253.160.0/19

11.253.192.0/19

11.1.1.0/24

11.1.2.0/24

11.1.253.0/24

11.1.254.0/24

11.1.253.32/27

11.1.253.64/27

11.1.253.160/27

11.1.253.192/27

VLSM permits the recursive division of a netrwork prefix

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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.

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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?

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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.

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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 = 00001011.0000001.00000010.00000101

* 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