Chapter 4: VLSM and Classless Inter Domain Routing. ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved.

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1 Chapter 4: VLSM and Classless Inter Domain Routing 1

2 What will we Learn from chapter 4? 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

3 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 the Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR) CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space. -VLSM is simply subnetting a subnet 3

4 Classful and Classless IP Addressing Classful IP addressing 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

5 Classful and Classless IP Addressing The High Order Bits These are the leftmost bits in a 32 bit address 5

6 Classful and Classless IP Addressing Classes of IP addresses are identified by the decimal number of the 1st octet Class A address begin with a 0 bit Range of class A addresses = to Class B address begin with a 1 bit and a 0 bit Range of class B addresses = to Class C addresses begin with two 1 bits & a 0 bit Range of class C addresses = to

7 Classful and Classless IP Addressing The IPv4 Classful Addressing Structure (RFC 790) 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 7

8 Classful and Classless IP Addressing 8

9 Classful and Classless IP Addressing Purpose of a subnet mask It is used to determine the network portion of an IP address 9

10 Classful and Classless IP Addressing Classful Routing Updates -Recall that classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing updates The reason is that the Subnet mask is directly related to the network address 10

11 Classful and Classless IP Addressing Classless Inter-domain Routing (CIDR RFC 1517) Advantage of CIDR : -More efficient use of IPv4 address space -Route summarization Requires subnet mask to be included in routing update because address class is meaningless Recall purpose of a subnet mask: -To determine the network and host portion of an IP address 11

12 Classful and Classless IP Addressing Classless IP Addressing CIDR & Route Summarization -Variable Length Subnet Masking (VLSM) -Allows a subnet to be further sub-netted according to individual needs -Prefix Aggregation a.k.a. Route Summarization -CIDR allows for routes to be summarized as a single route 12

13 Classful and Classless IP Addressing Classless Routing Protocol Characteristics of classless routing protocols: -Routing updates include the subnet mask -Supports VLSM Supports Route Summarization 13

14 Classful and Classless IP Addressing Classless Routing Protocol Routing Protocol Routing updates Include subnet Mask Supports VLSM Ability to send Supernet routes Classful No No No Classless Yes Yes Yes 14

15 VLSM Classful routing -only allows for one subnet mask for all networks VLSM & classless routing -This is the process of subnetting a subnet -More than one subnet mask can be used -More efficient use of IP addresses as compared to classful IP addressing 15

16 VLSM VLSM the process of subnetting a subnet to fit your needs -Example: Subnet /16, 8 more bits are borrowed again, to create 256 subnets with a /24 mask. -Mask allows for 254 host addresses per subnet -Subnets range from: / 24 to / 24 16

17 Classless Inter-Domain Routing (CIDR) Route summarization done by CIDR -Routes are summarized with masks that are less than that of the default classful mask -Example: / 13 is the summarized route for the / 16 to / 16 classful networks 17

18 Classless Inter-Domain Routing (CIDR) 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 18

19 CCNA 3/Module 1 An Introduction to Classless Routing 19

20 Overview: Classful/Classless Routing Classful routing - a network must use the same subnet mask for the entire network Network IP Network Subnet Mask Classless routing using more than one subnet mask for a network address subnetting a subnet Network IP Network Subnet Masks

21 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. 21

22 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. 22

23 Overview: Scalability & Routing Tables Maximum theoretical routing table size is 60,000 entries. Classful addressing would have hit this capacity by mid Internet growth would have ended. 23

24 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 24

25 What is VLSM and why is it used? Classful routing protocols use one subnet mask for a single network Ex: , must use subnet mask VLSM allows a single autonomous system to have networks with different subnet masks, for example: Use a 30-bit subnet mask on network connections ( ) Use a 24-bit subnet mask for user networks up to 250 users ( ) Use a 22-bit subnet mask for user networks up to 1000 users ( ) 25

26 A waste of space In classless routing, recommended that first and last subnets have special use; not be used for host addresses First (SN 0) had same address for the network and subnet Last subnet (all-1 s) was the broadcast Address depletion has lead to use of these subnets Now acceptable practice to use the first and last subnets in conjunction with VLSM 26

27 A waste of space Network Address Borrow 3 bits = SNM Subnets = (2^H) 0, 32, 64, 96, 128, 160, 192,

28 A waste of space Network Address Borrow 3 bits = SNM Subnets = (2^H) 0, 32, 64, 96, 128, 160, 192, 224 If subnet zero is used, there are 8 useable subnets Each subnet can support 30 hosts Cisco routers use subnet zero by default IOS v If no ip subnet-zero command is used on the router, there are 7 useable subnets with 30 hosts per subnet If supporting 4 routers (1 subnet each) that need 3 WAN 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 28

29 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 29

30 When to use VLSM Networking design addressing scheme that allows: Growth Doesn t waste addresses on point-to-point links 30

31 When to use VLSM VLSM addressing applied instead results in: Variable sized subnets Take 1 of the 3 subnets and subnet it again Example (last subnet) Apply a 30 bit mask ( ) Creates a possible 8 ranges of addresses with 30 bits Best solution for point-to-point links use 2 host addresses instead of 30 31

32 Calculating subnets with VLSM VLSM helps to manage IP addresses VLSM can use one SNM for a point-to-point link and one SNM for a LAN 32

33 Calculating subnets with VLSM Foster sfabulousfilms 2 routers 1 in Hollywood (100 hosts) 1 in Ravenna (50 hosts) 1 WAN link (2 needed) IP/NW Address: Class C Use the BIGGEST first:

34 Calculating subnets with 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 /20 (would accommodate 4094 hosts) Binary = SNM = VLSM address /26 (needed for 62 hosts) Binary = SNM = If /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 (2 6 ) each supporting 62 hosts 34

35 Calculating Subnets w/vlsm Procedure to subnet a subnet /20 to /26 using VLSM: 1. Write in binary form Binary = Draw a vertical line between the 20 th and 21 st bits (the original subnet boundary) 3. Draw a vertical line between the 26 th and 27 th 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 35

36 Calculating Subnets w/vlsm Keep in mind that only unused subnets can be further subnetted If any address for a subnet is used cannot be further subnetted 36

37 Route Aggregation w/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 aggregation by using Classless InterDomain Routing (CIDR) Router needs to hold only one route /23 37

38 Route Aggregation w/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 only if the routers of a network run a classless routing protocol such as OSPF or EIGRP IP address and bit mask included in routing updates The summary route uses a prefix common to all addresses of an organizational group 38

39 Route Aggregation w/vlsm Carefully assign addresses in a hierarchical fashion to share same high-order bits for summarization A router: Must know subnets attached in detail Does not need to tell other routers about subnets 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 39

40 Configuring VLSM If VLSM is chosen, it must be configured correctly Example: One router has to support 60 hosts, needs 6 bits in host portion of address to provide 62 possible address (2 6 = 64 2 = 60) /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 (2 5 = 32 2 = 30) /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 (2 4 = 16 2 = 14) /28 (leaves 4 bits for hosts) /28 (leaves 4 bits for hosts) 40

41 Configuring VLSM Point-to-point connections are: /30 (2 address required, 2 bits = 2 host addresses) /30 (2 address required, 2 bits = 2 host addresses) /30 (2 address required, 2 bits = 2 host addresses) Choices = Configuration of the /30 subnet (.136/30 - network address;.137/30 and 138/30 host addresses.139/30 - broadcast address; : (config)#interface serial 0 (config-if)#ip address (config)#interface serial1 (config-if)#ip address

42 RIP History 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 Interior Gateway Protocol Routing protocol used between Autonomous Systems is an Exterior Gateway Protocol RIP v1: is an IGP that is classful 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 42

43 RIP History 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 or /8 Class B default classful mask is or /16 Class C default classful mask is or /24 43

44 RIP v2 Features 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 address for better efficiency Provides authentication in updates Clear text - default MD5 encryption typically used to encrypt enable secret passwords (Message-Digest 5) 44

45 Comparing RIP v1 & v2 Easy to configure RIP v1 Supports classful routing No subnet mask sent with routing updates (considered a limitation of v1) No authentication Uses hop count Easy to configure RIP v2 Supports classless routing Sends subnet mask with routing update Provides for authentication Uses hop count 16 hops as metric for infinite distance 16 hops as metric for infinite distance Broadcasts routing table updates Does not support prefix routing (all devices in same network must use same subnet mask) Multicasts updates Supports prefix routing (VLSM, different subnet masks can be used in same network) 45

46 Configuring RIP v2 To enable a dynamic routing protocol: 1. Select routing protocol FOSTER(config)#router rip FOSTER(config-router)#version 2 2. Configure routing protocol with the network IP address (identify physically connected network that will receive routing tables) FOSTER(config-router)#network FOSTER(config-router)#network Assign IP/SNM to interfaces 46

47 Verifying RIP v2 FOSTER#show ip protocols Shows protocol name Tells when updates are sent and when the next is due FOSTER#show ip route Tells if routers have learned about a newly added network Displays IP routing table FOSTER#show ip interface brief FOSTER#show running-config Summary of information status of interface Checks for a misconfigured routing protocol 47

48 Verifying RIP v2 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 48

49 1.2.6 Troubleshooting RIP v2 Foster# debug ip rip Foster# no debug all Foster# u all (undebug all) Displays RIP routing updates as they are sent and received Turns off all debugging 49

50 Default Routes 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 50

51 1.2.7 Default Routes Default Route Command: FOSTER(config)# ip route Next hop router Default NW 24 bit prefix 51

52 1.2.7 Default Routes DYNAMIC PROTOCOL Default Route Command FOSTER(config)# ip default-network Used to: Default NW 1. Give packets that are not in the routing table a place to go typically a router that connects to the Internet 2. Connect a router with a static default route 52

53 What did I Learn from chapter 4? Classful IP addressing IPv4 addresses have 2 parts: -Network portion found on left portion of an IP address -Host portion found on right portion of an IP address 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 conserve address space 53

54 What did I Learn from chapter 4? (con t) Classful Routing Updates Subnet masks are not sent in routing updates Classless IP addressing Benefit of classless IP addressing Can create additional network addresses using a subnet mask that fits your needs Uses Classless Interdomain Routing (CIDR) 54

55 What did I Learn from chapter 4? (con t) CIDR Uses IP addresses more efficiently through use of VLSM -VLSM is the process of subnetting a subnet Allows for route summarization -Route summarization is representing multiple contiguous routes with a single route 55

56 What did I Learn from chapter 4? (con t) Classless Routing Updates Subnet masks are included in updates 56