Objectives. Introduction. Classful IP Addressing. IPv4 Classful Addressing Structure (RFC 790)

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1 1 Objectives VLSM and CIDR Routing Protocols and Concepts Chapters 6 and 7 Compare and contrast classful and classless IP addressing. Review VLSM and explain the benefits of classless IP addressing. Describe the role of the Classless Inter- Domain Routing (CIDR) standard in making efficient use of scarce IPv4 addresses 2 Introduction Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the address In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classes IP address space was depleting rapidly In 1993 Classless Inter-Domain Routing (CIDR) CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space. VLSM is simply subnetting a subnet 3 As of January 2007, there are over 433 million hosts on Internet Initiatives to conserve IPv4 address space include: -VLSM & CIDR notation (1993, RFC 1519) -Network Address Translation (1994, RFC 1631) -Private Addressing (1996, RFC 1918) 4 IPv4 Classful Addressing Structure (RFC 790) Classful IP Addressing An IP address has 2 parts: The network portion Found on the left side of an IP address The host portion Found on the right side of an IP address The High Order Bits are the leftmost bits in a 32 bit address 5 Class A addresses = to Class B addresses = to Class C addresses = to

2 Classful IP Addressing Classful IP Subnetting 7 Purpose of a subnet mask It is used to determine the network portion of an IP address Classful routing updates do not include subnet masks Receiving router must assume the mask: - If the advertised network is part of the same major network as the interface it arrives on, use the same subnet mask as the interface - Else use the default mask for the class 8 is a Classful Routing Protocol Rules for Classful Subnetting A major network is subnetted using a single subnet mask so that all of its subnets are the same size The subnets of a major network must be contiguous Example: / / / Classless IP Addressing Classless Inter-domain Routing (CIDR RFC 1517) Advantages of CIDR More efficient use of IPv4 address space Route summarization smaller routing tables Requires subnet mask to be included in the routing updates because address class is meaningless Recall purpose of subnet mask To determine the network and host portion of an IP address 11 CIDR & Route Summarization CIDR allows for routes to be summarized as a single route 12

3 CIDR & Route Summarization Route Summarization is also called Prefix Aggregation / / / /21 Classless Routing Protocol Characteristics of classless routing protocols: - Routing updates include the subnet mask - Supports VLSM - Supports Route Summarization /20 route is / / / / / / /16 Steps to calculate a route summary List networks in binary format Count number of left most matching bits to determine summary route s mask Copy the matching bits and add zero bits to determine the summarized network address /13 Lab 6.4.1: Basic VLSM Addressing Scheme Lab 6.4.4: Basic Route Summarization 17 18

4 Routing Protocols To use CIDR, VLSM, discontiguous subnets You cannot run a classful routing protocol, which means you cannot run or IGRP All other routing protocols covered on this course are classless routing protocols, i.e.:, EIGRP, OSPF, IS-IS, BGP We now look at 19 Difference between & A classful distance vector routing protocol Does not support discontiguous subnets Does not support VLSM Does not send subnet mask in routing update Routing updates are broadcast Automatically summarizes routes at class boundary (this cannot be disabled) is the default 20 Difference between & A classless distance vector routing protocol that is an enhancement of s features. Subnet mask and next hop address is included in updates Routing updates are multicast (uses less bandwidth) Automatic summarization of routes can be disabled The use of authentication is an option 21 Similarities between & Use of hold-down timers to prevent routing loops Use of split horizon or split horizon with poison reverse Use of triggered updates Hop count used as metric Maximum hop count of 15 Both do automatic summarization of routes at the class boundary by default 22 Comparing & Message Formats Message format is similar to but includes subnet mask and next hop address fields A configured router will: Accept updates from routers but will not recognize the subnet mask information Will only send updates 23 24

5 Lab Basic Configuration 25 Lab Basic Configuration Configure the 3 routers using Do this by pasting in the prepared configuration files Observe the problem on R2 due to discontiguous subnets of int fa0/0 ip address int s0/0 ip address int s0/1 ip address clock rate router rip passive-interface fa0/0 network network Lab Basic Configuration of and To solve the problem Configure Disable autosummary int fa0/0 ip address int s0/0 ip address int s0/1 ip address clock rate router rip version 2 no auto-summary passive-interface fa0/0 network network Routing Protocol Distance Vector Classless Routing Protocol Uses Hold- Down Timers Use of Split Horizon or Split Horizon w/ Poison Reverse Max Hop count = 15 Auto (by default) Support CIDR Supports VLSM Uses Authentication Classful IP addressing IPv4 addresses have 2 parts: Network & Host Class A, B, & C addresses were designed to provide IP addresses for different sized organizations The class of an IP address is determined by the decimal value found in the 1 st octet IP addresses are running out so the use of Classless Inter Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM) are used to try and conserve address space Classful Routing Updates Subnet masks are not sent in routing updates Benefits of classless IP addressing Can create additional network addresses using a subnet mask that fits your needs Uses Classless Interdomain Routing (CIDR) 29 30