Planning for Information Network

Planning for Information

Network

Lecture 6:

Planning the IP Addressing

Hierarchy

Dr. Mehdi Ebady Manaa 1

Hierarchical Addressing

** Each IP address is divided into a prefix and a suffix.

- Prefix identifies network to which computer is

attached

- Suffix identifies computer within that network

** Address format makes routing efficient

Dr. Mehdi Ebady Manaa 2

Hierarchical Addressing

The IP addressing scheme is hierarchical, and IP

routers make hierarchical decisions.

Recall that an IP address comprises a prefix part

and a host part (suffix).

A router has to know only how to reach the next

hop; it does not have to know the details of how to

reach an end node that is not local.

Routers use the prefix to determine the path for a

destination address that is not local. The host part is

used to reach local hosts.

Dr. Mehdi Ebady Manaa 3

Route Summarization

With route summarization, also referred to as

route aggregation or supernetting, one route in the

routing table represents many other routes.

Summarizing routes reduces the routing update

traffic and reduces the number of routes in the routing

table and overall router overhead in the router receiving

the routes.

In a hierarchical network design, effective use of

route summarization can limit the impact of topology

changes to the routers in one section of the network.

Dr. Mehdi Ebady Manaa 4

CIDR

Classless Inter-Domain Routing (CIDR) is a

mechanism developed to help alleviate the problem of IP address

exhaustion and growth of routing tables.

The idea behind CIDR is that blocks of multiple addresses

(for example, blocks of Class C address) can be combined, or

aggregated, to create a larger (that is, more hosts allowed)

classless set of IP addresses. Blocks of Class C network numbers

are allocated to each network service provider; organizations

using the network service provider for Internet connectivity are

allocated subsets of the service provider’s address space as

required. These multiple Class C addresses can then be

summarized in routing tables, resulting in fewer route

advertisements.

The CIDR mechanism can be applied to blocks of Class

A, B, and C addresses; it is not restricted to Class C.)

Dr. Mehdi Ebady Manaa 5

Route Summarization

For summarization to work correctly, the following

requirements must be met:

■ Multiple IP addresses must share the same leftmost bits.

■ Routers must base their routing decisions on a 32-bit IP address

and a prefix length of up to 32 bits.

■ Routing protocols must carry the prefix length with the 32-bit

IP address.

Dr. Mehdi Ebady Manaa 6

Route Summarization - Example

For example, assume that a router has the following networks

behind it:

192.168.168.0/24

192.168.169.0/24

192.168.170.0/24

192.168.171.0/24

192.168.172.0/24

192.168.173.0/24

192.168.174.0/24

192.168.175.0/24

Each of these networks could be advertised separately; however,

this would mean advertising eight routes. Instead, this router can

summarize the eight routes into one route and advertise 192.168.168.0/21.

By advertising this one route, the router is saying, “Route packets

to me if the destination has the first 21 bits the same as the first 21 bits of

192.168.168.0.”

Dr. Mehdi Ebady Manaa 7

Route Summarization - Example

The following figure illustrates how this summary route is determined.

The addresses all have the first 21 bits in common and include all the

combinations of the other 3 bits in the network portion of the address;

therefore, only the first 21 bits are needed to determine whether the

router can route to one of these specific addresses.

Dr. Mehdi Ebady Manaa 8

192.168.168.0 =

192.168.169.0 =

192.168.170.0 =

192.168.171.0 =

192.168.172.0 =

192.168.173.0 =

192.168.174.0 =

192.168.175.0 =

11000000 10101000 10101 000 00000000

11000000 10101000 10101 001 00000000

11000000 10101000 10101 010 00000000

11000000 10101000 10101 011 00000000

11000000 10101000 10101 100 00000000

11000000 10101000 10101 101 00000000

11000000 10101000 10101 110 00000000

11000000 10101000 10101 111 00000000

Number of Common Bits = 21

Number of Non-Common Network Bits = 3

Number of Host Bits = 8

Route Summarization - Example

Dr. Mehdi Ebady Manaa 9

Router D can either send four routing update entries or summarize the four addresses into a

single network number. If router D summarizes the information into a single network number

entry, the following things happen:

** Bandwidth is saved on the link between routers D and E.

** Router E needs to maintain only one route and therefore saves memory.

** Router E also saves CPU resources, because it evaluates packets against fewer entries in

its routing table.

Benefits of Hierarchical Addressing

A network designer decides how to implement the IP

addressing hierarchy based on the network’s size, geography, and

topology. In large networks, hierarchy within the IP addressing

plan is mandatory for a stable network (including stable routing

tables). For the following reasons, a planned, hierarchical IP

addressing structure, with room for growth, is recommended for

networks of all sizes:

1- Influence of IP addressing on routing: An IP addressing

plan influences the network’s overall routing. Before allocating

blocks of IP addresses to various parts of the network and

assigning IP addresses to devices, consider the criteria for an

appropriate and effective IP addressing scheme. Routing stability,

service availability , and network scalability are some crucial and

preferred network characteristics and are directly affected by IP

address allocation and deployment.

Dr. Mehdi Ebady Manaa 10

Benefits of Hierarchical Addressing

Dr. Mehdi Ebady Manaa 11

2- Modular design and scalable solutions: Whether building a

new network or adding a new service on top of an existing

infrastructure, a modular design helps to deliver a long-term,

scalable solution. IP addressing modularity allows the aggregation

of routing information on a hierarchical basis.

3- Route aggregation: Route aggregation is used to reduce routing

overhead and improve routing stability and scalability. However, to

implement route aggregation, a designer must be able to divide a

network into contiguous IP address areas and must have a solid

understanding of IP address assignment, route aggregation, and

hierarchical routing.

Summarization Groups

Dr. Mehdi Ebady Manaa 12

To reduce the routing overhead in a large network, a multilevel

hierarchy might be required. The depth of hierarchy depends on

the network size and the size of the highest-level summarization

group. The following figure shows an example of a network

hierarchy.

Summarization Groups

A typical organization has up to three levels of

hierarchy:

■ First level: Network locations typically represent the first level of

hierarchy in enterprise networks. Each location typically represents a

group of summarized subnets, known as a summarization group.

■ Second level: A second level of hierarchy can be done within first-

level summarization groups. For example, a large location can be

divided into smaller summarization groups that represent the buildings

within that location. Not all first-level summarization groups require a

second level of hierarchy.

■ Third level: To further minimize the potential routing overhead and

instability, a third level of hierarchy can exist within the second-level

summarization group. For example, sections or floors within

individual buildings can represent the third-level summarization

group.

Dr. Mehdi Ebady Manaa 13

Impact of Poorly Designed IP Addressing

A poorly designed IP addressing scheme usually results in IP

addresses that are randomly assigned on an as-needed basis. In this

case, the IP addresses are most likely dispersed through the network

with no thought as to whether they can be grouped or summarized. A

poor design provides no opportunity for dividing the network into

contiguous address areas, and therefore no means of implementing

route summarization.

The next figure is a sample network with poorly designed IP

addressing; it uses a dynamic routing protocol. Suppose that a link in

the network is flapping (changing its state from UP to DOWN, and

vice versa) ten times per minute. Because dynamic routing is used, the

routers that detect the change send routing updates to their neighbors,

those neighbors send it to their neighbors, and so on. Because

aggregation is not possible, the routing update is propagated

throughout the entire network, even if there is no need for a distant

router to have detailed knowledge of that link.

Dr. Mehdi Ebady Manaa 14

Impact of Poorly Designed IP Addressing

Dr. Mehdi Ebady Manaa 15

A Poorly Designed IP Addressing Scheme Results in Excess Routing Traffic

Impact of Poorly Designed IP Addressing

Impacts of poorly designed IP addressing include the

following:

■ Excess routing traffic consumes bandwidth: When any route

changes, routers send routing updates. Without summarization,

more updates are sent, and the routing traffic consumes more

bandwidth.

■ Increased routing table recalculation: Routing updates

require routing table recalculation, which affects the router’s

performance and ability to forward traffic.

■ Possibility of routing loops: When too many routing changes

prevent routers from converging with their neighbors, routing

loops might occur, which might have global consequences for an

organization.

Dr. Mehdi Ebady Manaa 16

Benefits of Route Aggregation

Implementing route aggregation on border routers

between contiguously addressed areas controls routing table size.

The following figure shows an example of implementing

route summarization (aggregation) on the area borders in a

sample network. If a link within an area fails, routing updates are

not propagated to the rest of the network, because only the

summarized route is sent to the rest of the network, and it has not

changed; the route information about the failed link stays within

the area. This reduces bandwidth consumption related to routing

overhead and relieves routers from unnecessary routing table

recalculation.

Efficient aggregation of routing advertisements narrows

the scope of routing update propagation and significantly

decreases the cumulative frequency of routing updates.

Dr. Mehdi Ebady Manaa 17

Benefits of Route Aggregation

Dr. Mehdi Ebady Manaa 18

• A Hierarchical IP Addressing Plan Results in Reduced Routing Traffic

Fixed- and Variable-Length Subnet Masks

*Another consideration when designing the IP addressing

hierarchy is the subnet mask to use either the same mask for the

entire major network or different masks for different parts of the

major network.

*Some routing protocols require FLSM; others allow VLSM.

**A major network is a Class A, B, or C network.

**Fixed-Length Subnet Masking (FLSM) is when all subnet

masks in a major network must be the same.

**Variable-Length Subnet Masking (VLSM) is when subnet

masks within a major network can be different. In modern

networks, VLSM should be used to conserve the IP addresses.

Dr. Mehdi Ebady Manaa 19

FLSM

FLSM requires that all subnets of a major network have the same subnet

mask, which therefore results in less efficient address space allocation. For

example, in the network shown in the following figure, network 172.16.0.0/16

is subnetted using FLSM. Each subnet is given a /24 mask. The network is

composed of multiple LANs that are connected by point-to-point WAN links.

Because FLSM is used, all subnets have the same subnet mask. This is

inefficient, because even though only two addresses are needed on the point-

to-point links, a /24 subnet mask with 254 available host addresses is used.

Dr. Mehdi Ebady Manaa 20

VLSM

VLSM makes it possible to subnet with different

subnet masks and therefore results in more efficient

address space allocation. VLSM also provides a greater

capability to perform route summarization, because it

allows more hierarchical levels within an addressing

plan. VLSM requires prefix length information to be

explicitly sent with each address advertised in a routing

update.

Dr. Mehdi Ebady Manaa 21

VLSM

For example, in the network shown in the following figure, network

172.16.0.0/16 is subnetted using VLSM. The network is composed of multiple

LANs that are connected by point-to-point WAN links. The point-to-point

links have a subnet mask of /30, providing only two available host addresses,

which is all that is needed on these links. The LANs have a subnet mask of /24

because they have more hosts that require addresses.

Dr. Mehdi Ebady Manaa 22

Routing Protocol Considerations

To use VLSM, the routing protocol in use must be

classless.

Classful routing protocols permit only FLSM.

Dr. Mehdi Ebady Manaa 23

Classful Routing Protocols

The following rules apply when classful routing protocols

are used:

■ The routing updates do not include subnet masks.

■ When a routing update is received and the routing information is

about one of the following:

— Routes within the same major network as configured on the

receiving interface, the subnet mask configured on the receiving

interface is assumed to apply to the received routes also. Therefore, the

mask must be the same for all subnets of a major network. In other

words, subnetting must be done with FLSM.

— Routes in a different major network than configured on the receiving

interface, the default major network mask is assumed to apply to the

received routes. Therefore, automatic route summarization is performed

across major network (Class A, B, or C) boundaries, and subnetted

networks must be contiguous.

Dr. Mehdi Ebady Manaa 24

Classful Routing Protocols

Dr. Mehdi Ebady Manaa 25

Classful Routing Protocols

Dr. Mehdi Ebady Manaa 26

The following figure illustrates a sample network with a

discontiguous 172.16.0.0 network that runs a classful routing

protocol. Routers A and C automatically summarize across the

major network boundary, so both send routing information about

172.16.0.0 rather than the individual subnets (172.16.1.0/24 and

172.16.2.0/24).

Consequently, Router B receives two entries for the

major network 172.16.0.0, and it puts both entries into its routing

table. Router B therefore might make incorrect routing decisions.

Because of these constraints, classful routing is not often

used in modern networks. Routing Information Protocol (RIP)

version 1 (RIPv1) is an example of a classful routing protocol.

Classful Routing Protocols

Dr. Mehdi Ebady Manaa 27

Classful Routing Protocols Do Not Send the Subnet Mask in the Routing

Update

Classless Routing Protocols

Dr. Mehdi Ebady Manaa 28

The following rules apply when classless routing protocols

are used:

■ The routing updates include subnet masks.

■ VLSM is supported.

■ Automatic route summarization at the major network boundary is not

required, and route summarization can be manually configured.

■ Subnetted networks can be discontiguous.

Consequently, all modern networks should use classless routing. Examples of

classless routing protocols include RIP version 2 (RIPv2), Enhanced Interior

Gateway Routing Protocol (EIGRP), and Border Gateway Protocol (BGP).

Classless Routing Protocols

Dr. Mehdi Ebady Manaa 29

Classless Routing Protocols

Dr. Mehdi Ebady Manaa 30

The following figure illustrates how discontiguous

networks are handled by a classless routing protocol.

Within this network, the classless routing protocol is

running that does not automatically summarize at the network

boundary. In this example, Router B learns about both

subnetworks 172.16.1.0/24 and 172.16.2.0/24, one from each

interface; routing is performed correctly.

Classless Routing Protocols

Dr. Mehdi Ebady Manaa 31

Classless Routing Protocols Send the Subnet Mask in the Routing Update

Assessment

1- With classful routing, routing updates _ _ _ _ carry

the subnet mask.

2- With classless routing, routing updates _ _ _ _carry

the subnet mask.

A- Do

B- Do not

Dr. Mehdi Ebady Manaa 32

Thank you

Dr. Mehdi Ebady Manaa 33