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CCNA 3/Module 1. Introduction to Classless Routing . Overview: Classful/Classless Routing. Classful routing - a network must use the same subnet mask for the entire network. Classless routing – using more than one subnet mask for a network address “subnetting a subnet” .

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Ccna 3 module 1 l.jpg

CCNA 3/Module 1

Introduction to

Classless Routing


Overview classful classless routing l.jpg
Overview: Classful/Classless Routing

  • Classful routing - a network must use the same subnet mask for the entire network

  • Classless routing – using more than one subnet mask for a network address

    • “subnetting a subnet”


Overview classful ipv4 addressing limits l.jpg
Overview: (Classful) IPv4 Addressing Limits

  • IPv4 – 20 years old

  • IPv4 – even with subnetting, couldn’t handle the global demand for Internet connectivity

  • Class B space was on the verge of depletion.

  • Rapid and substantial increase in the size of the Internet's routing tables.

    • As more Class C's came online, the flood of new network information threatened Internet routers' capability to cope.


Overview classful ipv4 addressing limits4 l.jpg
Overview: (Classful) IPv4 Addressing Limits

  • Provides IP scheme with limitations:

    • Class A – 126 networks: 16,777,214 hosts each

    • Class B – 65,000 networks: 65,534 hosts each

    • Class C – 2 million networks: 254 hosts each

  • While available addresses were running out, only 3% of assigned addresses were

    actually being used!

    • Subnet zero, broadcast addresses,

      pool of unused addresses at

      Class A and B sites, etc.


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Overview: Scalability & Routing Tables

  • Maximum theoretical routing table size is 60,000 entries.

    • Classful addressing would have hit this capacity by mid-1994.

    • Internet growth would have ended.


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1.1.1 What is VLSM and why is it used?

  • The purpose of VLSM is to alleviate the shortage of IP addresses

  • VLSM allows:

    • More than one subnet mask within the same NW

    • Or . . . Multiple SNMasks with ONE IP Address

      • Use of long mask on networks with few hosts

      • Use of short mask on networks with many hosts

  • In order to use VLSM, the routing protocol must support it.

    • Cisco routers with the following routing protocols support VLSM:

      • OSPF (Open Shortest Path First)

      • IS-IS (Integrated Intermediate System to Intermediate System)

      • EIGRP (Enhanced Interior Gateway Routing Protocol)

      • RIP v2

      • Static Routing


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1.1.1 What is VLSM and why is it used?

Classfulrouting protocols use one subnet mask for a single network

  • Ex: 192.168.187.0, must use subnet mask 255.255.255.0

    VLSMallows a single autonomous system to have networks withdifferent subnet masks, for example:

  • Use a 30-bit subnet mask on network connections

    • (255.255.255.252)

  • Use a 24-bit subnet mask for user networks up to 250 users

    • (255.255.255.0)

  • Use a 22-bit subnet mask for user networks up to 1000 users

    • (255.255.252.0)


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1.1.2 A waste of space

  • In classless routing, it was recommended that firstand lastsubnet not be used

    • First (SN 0) had same address for the network and subnet

    • Last subnet (all-1’s) was the broadcast

    • Always could have been used, was not recommended practice

    • Address depletion has lead to use of these subnets

    • Now acceptable practice to use the first and last subnets in conjunction with VLSM



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1.1.2 A waste of space

If subnet zero is used, there are 8useable subnets

  • Each subnet can support 30 hosts

  • Cisco routers use subnet zero by default IOS v. 12.0+

    If no ip subnet-zero command is used on the router, there are 7useable subnets with 30 hosts per subnet

  • If supporting 4 routers (1 subnet each) that need3WAN links to each other, all subnets are used

    • No room for growth

    • Waste of 28 host addresses for each WAN (point-to-point) links or 1/3 of potential address space


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    1.1.2 A waste of space

    FOSTER(config)#no ip subnet-zero

    • Disables the capability to use subnets that include the network address of the unsubnetted network


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    1.1.3 When to use VLSM

    Design addressing scheme that allows:

    • Growth

    • Doesn’t waste addresses on point-to-point links

    • VLSM addressing applied instead results in:

      • Variable sized subnets

        • Take 1 of the 3 subnets and subnet it again

        • Example 192.168.187.224(last subnet)

          • Apply a 30 bit mask (225.225.225.252)

          • Creates a possible8 rangesof addresses with30 bits

      • Best solution forpoint-to-point links – use 2 host addresses instead of 30


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    1.1.4 Calculating subnets with VLSM use VLSM

    VLSM helps to manage IP addresses

    • VLSM can use one SNM for a point-to-point link and

      one SNM for a LAN


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    1.1.4 Calculating subnets with VLSM use VLSM

    Foster’sFabulousFilms

    • 2 routers

      • 1 in Hollywood (100 hosts)

      • 1 in Ravenna (50 hosts)

      • 1 WAN link (2 needed)

    • IP/NW Address: 192.16.10.0

      • Class C

    • Use the BIGGEST first:

    • 100

    • 50

    • 2


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    1.1.4 Calculating subnets with VLSM use VLSM

    Foster’sFabulousFilms

    • 2 routers

      • 1 in Hollywood (100 hosts)

      • 1 in Ravenna (50 hosts)

      • 1 WAN link (2 needed)

    • IP/NW Address: 192.16.10.0

      • Class C

    • Use the BIGGEST first:

    • 100 /25

    • 50 /26

    • 2 /30


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    1.1.4 Calculating subnets with VLSM use VLSM

    If VLSM were used instead of classful routing:

    • A 24-bit mask could be used for LAN segments for 250 hosts

    • A 30-bit mask could be used for WAN segments for 2 hosts

    • 172.16.32.0/20 (would accommodate 4094 hosts)

      • Binary = 10101100.00010000.00100000.00000000

      • SNM = 11111111.11111111.11110000.00000000

    • VLSM address172.16.32.0/26 (needed for 62 hosts)

      • Binary = 10101100.00010000.00100000.00000000

      • SNM = 11111111.11111111.11111111.11000000

    • If 172.16.32.0/20 used, but only 10 hosts on segment, would provide 4094 hosts and waste 4084 addresses

    • By further subnetting /20 to /26, gain 64 subnets (26) each supporting 62 hosts


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    1.1.4 Calculating Subnets w/VLSM use VLSM

    Procedure to subnet a subnet /20 to /26 using VLSM:

    1. Write 172.16.32.0 in binary form

    • Binary = 10101100.00010000.00100000.00000000

  • Draw a vertical line between the 20th and 21st bits (the original

    subnet boundary)

    3. Draw a vertical line between the 26th and 27th bits extending the bits to segment/host needs

    4. Calculate the number of subnet addresses between the two vertical lines (lowest to highest) in value


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    1.1.4 Calculating Subnets w/VLSM use VLSM

    • Keep in mind that only unused subnets can be further subnetted

      • If any address for a subnet is used cannot be further subnetted


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    1.1.5 Route Aggregation w/VLSM use VLSM

    • Every network needs a separate entry in routing table

    • Each subnet needs a separate entry

    • Aggregation will reduce routing table size

    • When using VLSM keep subnetwork numbers grouped together in the network to allow for aggregationby using Classless InterDomain Routing(CIDR)

      • 172.16.14.0

      • 172.16.15.0

      • Router needs to carry only one route 172.16.14.0/23


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    1.1.5 Route Aggregation w/VLSM use VLSM

    • Using CIDR and VLSM prevents address waste and promotes route aggregation or summarization

      • Without summarization, Internet would collapse

    • Summarization reduces burden on upstream routers

    • This process of summarization continues until entire network is advertised as a single aggregate route

    • Summarization is also called supernetting

    • Possible if the routers of a network run a classless routing protocol such as OSPF or EIGRP

      • Consists of IP address and bit mask in routing updates

      • The summary route uses prefix common to all addresses of organization


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    1.1.5 Route Aggregation w/VLSM use VLSM

    Carefully assign addresses in a hierarchical fashion to share same high-order bits for summarization

    • A router must know subnets attached in detail

    • A router does not need to tell other routers about subnets

    • A router using aggregate routes has fewer entries in routing table

  • VLSM allows for summarization of routes

    • Works even if networks are not contiguous

  • VLSM increases flexibly by summarization on higher-order bits

    • Used to calculate the network number of the summary route

    • Uses only shared highest-order bits


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    1.1.6 Configuring VLSM use VLSM

    • If VLSM is chosen, it must be configured correctly

    • Example: 192.168.10.0

      • One router has to support 60 hosts, needs 6 bits in host portion of address to provide 62 possible address

        • (26 = 64 – 2 = 60)

          192.168.10.0/26(leaves 6 bits for hosts)

      • One router has to support 28 hosts, needs 5 bits in host portion of address to provide 30 possible hosts

        • (25 = 32 – 2 = 30) 192.168.10.64/27(leaves 5 bits for hosts)

      • Two routers have to support 12 hosts each, needs 4 bits in host portion of address to provide 14 possible hosts (24 = 16 – 2 = 14) 192.168.10.96/28(leaves 4 bits for hosts)

        192.168.10.112/28 (leaves 4 bits for hosts)


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    1.1.6 Configuring VLSM use VLSM

    • Point-to-point connections are:

      • 192.168.10.128/30 (2 address required, 2 bits = 2 host addresses)

      • 192.168.10.132/30 (2 address required, 2 bits = 2 host addresses)

      • 192.168.10.136/30 (2 address required, 2 bits = 2 host addresses)

        • Choices = .136 .137 .138 .139

    • Configuration as follows for the 192.168.10.136/30 network (.136/30 - network address;.139/30 - broadcast address; .137/30 and 138/30 – host addresses:

      • (config)#interface serial 0

      • (config-if)#ip address 192.168.10.137 255.255.255.252

      • (config)#interface serial1

      • (config-if)#ip address 192.168.10.138 255.255.255.252


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    1.2.1 RIP History use VLSM

    Internet is a collection of autonomous systems (AS)

    • Each AS is administered by a single entity

    • Each AS has its own routing technology

      Routing protocol used within AS is InteriorGateway Protocol

      Routing protocol used betweenAutonomous Systems is an Exterior Gateway Protocol

      RIP v1:

    • is an IGP that is classful

    • was designed to work within moderate-sized AS

    • is a distance vector routing protocol

    • by default, broadcasts entire routing table every 30 seconds

    • uses hop count as metric (16 max)

    • is capable of load balancing 6 equal-cost paths (4 default)

    • Does not send subnet mask information in its updates

    • Is not able to support VLSM or CIDR


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    1.2.1 RIP History use VLSM

    If the router receives information about a network, and the receiving interface belongs to same network but is on a different subnet, the router applies the one subnet mask configured on the receiving interface

    • Class A default classful mask is 255.0.0.0

    • Class B default classful mask is 255.255.0.0

    • Class C default classful mask is 255.255.255.0


    1 2 2 rip v2 features l.jpg
    1.2.2 RIP v2 Features use VLSM

    RIP v2 is an Improved version of RIP v1 with following features:

    • Distance vector protocol

    • Uses hop count as metric

    • Uses hold-down timers (prevent routing loops), default 180 sec.

    • Uses split horizon to prevent routing loops

    • Uses 16 hops as infinite distance

    • Provides prefix routing (sends subnet mask with route update)

    • Supports use of classless routing (VLSM)

    • Multicasts updates using 224.0.0.9 address for better efficiency

    • Provides authentication in updates

      • Clear text - default

      • MD5 encryption – typically used to encrypt enable secret passwords (Message-Digest 5)



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    1.2.4 Configuring RIP v2 use VLSM

    To enable a dynamic routing protocol:

    1. Select routing protocol

    • FOSTER(config)#router rip

    • FOSTER(config-router)#version 2

  • Configure routing protocol with the network IP address (identify physically connected network that will receive routing tables)

    • FOSTER(config-router)#network 10.0.0.0

    • FOSTER(config-router)#network 172.16.0.0

      3. Assign IP/SNM to interfaces



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    1.2.5 Verifying RIP v2 use VLSM

    • RIP updates table every 30 seconds

    • If no update received in 180 seconds, route marked as down

    • If no update after 240 seconds, removes from routing table entry



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    1.2.7 Default Routes use VLSM

    Three ways a router learns about paths:

    1. Static routes – manual configuration of routes (next hop)

    • Uses ip route command

      2. Default routes – manually defined path to take when there is no

      known route to a destination

      3. Dynamic routes – routers lean paths by receiving updates from

      other routers


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    1.2.7 Default Routes use VLSM

    Default Route Command:

    Default Route Command:

    FOSTER(config)# ip route 172.16.1.0255.255.255.0172.16.2.1

    FOSTER(config)# ip route 172.16.1.0255.255.255.0172.16.2.1

    Default NW

    Default NW

    Tells that 8 bits of subnetting in effect

    Tells that 8 bits of subnetting in effect

    Next hop router

    Next hop router


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    1.2.7 Default Routes use VLSM

    DYNAMIC PROTOCOL Default Route Command

    Used to:

    1. Give packets that are not ID’d in the routing table a place to go

    • Usually a router that connects to the Internet

      2. Connect a router with a static default route

    FOSTER(config)# ip default-network 192.168.20.0

    Default NW