day 5.5 subnetting.ppt

INTRODUCTION TO CLASSLESS ROUTING

CCNA v3.0 Semester 3

CLASSFUL ADDRESSINGThe original IPv4 address architecture used an 8 bit network number for Class A addresses, a 16 bit network number for Class B addresses, and a 24 bit network number for Class C addresses.*1 - 126128 - 191192 - 223


CLASSFUL ADDRESSINGClassful addressing (A, B, C, etc.) is basically obsolete.

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Class

The Class System

Address ClassApplicationNumber of Network BitsNumber of Host BitsDecimal Address RangeNumber of AddressesNumber of Possible Host

Class ALarge Networks8 bits24 bits1 - 12612616,777,214

Class BMedium-sized Networks16 bits16 bits128 - 19165,53465,534

Class CSmall Networks24 bits8 bits192 - 2232,097,152254

Prefix

Prefix Equivalents

Network PrefixEquivalent Number of Class AddressesNumber of Hosts

/271/8th of a Class C32


/251/2 of a Class C128

/241 Class C256

/232 Class C215

/224 Class C1,024

/218 Class C2,048

/2016 Class C4,096

/1932 Class C8,192

/1864 Class C16,384

/17128 Class C32,768

/16256 Class C or 1 Class B65,536


/141,024 Class C or 4 Class B262,144


/124096 Class C or 16 Class B1,048,576

/118192 Class C or 32 Class B2,097,152

/1016384 Class C or 64 Class B4,194,304

/932768 Class C or 128 Class B8,388,608

/865,536 Class C or 256 Class B or 1 Class A16,777,216

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WHAT IS VLSM?A Variable Length Subnet Mask (VLSM) is a means of allocating IP addressing resources to subnets according to their individual need rather than some general network-wide rule.

VLSM allows an organization to use more than one subnet mask within the same network address space. It is often referred to as subnetting a subnet, and can be used to maximize addressing efficiency.

Large subnets are created for addressing LANs and small subnets are created for WAN links (a 30 bit mask is used to create subnets with only two host).*


SUBNETTING VS. VLSM Subnetting allows you to divide big networks into smaller, equal-sized slices. VLSM allows you to divide big networks into smaller, different-sized slices. This enables you to make maximum use of your valuable IP address space. So basically, you are now utilizing subnet masks in the same IP address space.*


ROUTING PROTOCOLS SUPPORTING VLSMRIP v2EIGRPOSPF*


ADDRESSING A NETWORK WITH STANDARD SUBNETTINGSite A has two Ethernet networksSite B had one Ethernet networkSite C had one Ethernet network 207.21.24.0 /24

How many network addresses are needed?

How many hosts are needed for the largest LAN?

How many bits need to be borrowed to address this network?*


ADDRESSING A NETWORK WITH STANDARD SUBNETTINGSite A has two Ethernet networksSite B had one Ethernet networkSite C had one Ethernet network*If we borrow 3 bits from a class C address, that will give us eight networks, but we can only use six of them. Each network will have 30 usable addresses. It will take four network addresses to accommodate the Ethernet networks at each site. That leaves us with two extra networks. There is also a point-to-point WAN connection between each site. These two connections will take up the remaining two networks.


ADDRESSING A NETWORK WITH STANDARD SUBNETTINGBorrowing 3 bits will meet the current needs of the company, but it leaves little room for growth.Each network will have 30 usable addresses, including the point-to-point WAN links (which only require two addresses).

*207.21.24.0

Sheet1

Subnet #Subnet AddressBits Masked

0207.21.24.0/27

1207.21.24.32/27

2207.21.24.64/27

3207.21.24.96/27

4207.21.24.128/27

5207.21.24.160/27

6207.21.24.192/27

7207.21.24.224/27

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*We can use subnet 0To enable subnet 0 on a Cisco router (if not already enabled), it is necessary to use the global configuration command ip subnet-zero.Router# configure terminal (config t) Router(config)# ip subnet-zero

To disable subnet 0, use the no form of this command.Router# configure terminal Router(config)# no ip subnet-zero


*Subnetting in a Box0255To begin, in a class C network there are 256 addresses. When we subnet the address, we break it down in to smaller units or subnets.256 addresses


*Subnetting in a Box0255If we were to borrow 1 bit, it would break the 256 addresses in to two parts (networks) with each part (subnet) having 128 addresses.The subnet mask would be 255.255.255.128.


*Subnetting in a Box0255If we were to borrow 2 bits, it would break each of these 2 networks in half again. This would give us 4 networks, each with 64 addresses.The subnet mask would now be 255.255.255.192.


*Subnetting in a Box0255If we were to borrow 3 bits, it would break each of these 4 networks in half again. This would give us 8 networks, each with 32 addresses.The subnet mask would now be 255.255.255.224.


*Subnetting in a Box0255If we were to borrow 4 bits, it would break each of these 8 networks in half again. This would give us 16 networks, each with 16 addresses.The subnet mask would now be 255.255.255.240.31329596159160223224


ADDRESSING A NETWORK USING VLSMWhen using VLSM to subnet an address, not all of the subnets have to be the same size. A different subnet mask may be applied to some of the subnets to further subnet the address.In order to take advantage of VLSM, the proper routing protocol must be selected.Not all routing protocols share subnetting information in their routing table updates.

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Classful Routing Protocols(do not share subnet info)Classless Routing Protocols(do share subnet info)

RIP v1RIP v2

IGRPEIGRP

OSPF

IS-IS

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ADDRESSING A NETWORK USING VLSMTo begin subnetting this network using VLSM, identify the LAN with the largest number of hosts. Subnet the address 207.21.24.0 /24 based on this information. Site A has two Ethernet networks (25 hosts each)Site B had one Ethernet network (10 hosts)Site C had one Ethernet network (8 hosts)*

Sheet1

Subnet #Subnet AddressBits Masked

0207.21.24.0/27

1207.21.24.32/27

2207.21.24.64/27

3207.21.24.96/27

4207.21.24.128/27

5207.21.24.160/27

6207.21.24.192/27

7207.21.24.224/27

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ADDRESSING A NETWORK USING VLSMSubnet 1 & 2 can be used to address Site A Ethernet networks. Subnet 5 can be subnetted to accommodate Site B & C Ethernet networks. Subnet 6 can be subnetted to accommodate the WAN links.

*Free Addresses

Sheet1

Subnet #Subnet Address

0207.21.24.0 /27

1207.21.24.32 /27

2207.21.24.64 /27

3207.21.24.96 /27Sub-subnet 0207.21.24.160/28

4207.21.24.128 /27Sub-subnet 1207.21.24.176/28

5207.21.24.160 /27

6207.21.24.192 /27

7207.21.24.224 /27Sub-subnet 0207.21.24.192/30

Sub-subnet 1207.21.24.196/30

Sub-subnet 2207.21.24.200/30

Sub-subnet 3207.21.24.204/30

Sub-subnet 4207.21.24.208/30

Sub-subnet 5207.21.24.212/30

Sub-subnet 6207.21.24.216/30

Sub-subnet 7207.21.24.220/30

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ADDRESSING A NETWORK USING VLSMThrough applying VLSM, the topology was able to be addressed and still have two complete subnets available for future growth.

*207.21.24.32 /27207.21.24.64 /27207.21.24.160 /28207.21.24.176 /28207.21.24.192 /30207.21.24.196 /30


ADDRESSING A NETWORK USING VLSMEXERCISE 1Your company has been assigned IP network 195.39.71.0 /24. Given that headquarters (60 hosts) is connected to five branch offices (12 hosts each) by a WAN link, and to an ISP (the ISP owns the addresses on that link), determine an appropriate IP addressing scheme.*


*Given the IP address 195.39.71.0 /24, subnet according to the largest subnet needed. (Headquarters 60 hosts)0255You would need to borrow 2 bits or /26. This would give you 4 networks with 64 host addresses on each subnet.


*Playing it safe, we will not use the first subnet (subnet 0). We will start addressing with 195.39.71.64 /26. Headquarters needs 60 hosts, so we will assign them .64 - .127.

Headquarters60 hosts26 bit mask or /26(255.255.255.192)


*The 5 Branch offices only need 12 hosts each. The next address block available is the .128 - .191 block (64 addresses). Here we will apply VLSM.Headquarters60 hosts26 bit mask or /26(255.255.255.192)Using a /28 mask will give us 16 hosts at each location. This will take care of 4 of the Branch offices.


*To obtain a block for Branch 5, we will need to subnet the .192 - .255 block using a /28 mask.Headquarters60 hosts26 bit mask or /26(255.255.255.192)Branch 5 12 hosts /28 (255.255.255.240)


*Now we need to address the 5 WAN links that connect to the Branch offices. These are point-to-point connections and only require 2 addresses. 064128192Here we will use a /30 mask to further subnet the subnets.Headquarters60 hosts26 bit mask or /26(255.255.255.192)Branch 1 12 hosts /28 (255.255.255.240)Branch 2 12 hosts /28 (255.255.255.240)Branch 3 12 hosts /28 (255.255.255.240)Branch 4 12 hosts /28 (255.255.255.240)224208Branch 5 12 hosts /28 (255.255.255.240)WAN 5


*The remaining networks could be used for future growth of either LANs or WANs.Subnet 0 could also be further subnetted according to the needs of the network.064128192Headquarters60 hosts26 bit mask or /26(255.255.255.192)Branch 1 12 hosts /28 (255.255.255.240)Branch 2 12 hosts /28 (255.255.255.240)Branch 3 12 hosts /28 (255.255.255.240)Branch 4 12 hosts /28 (255.255.255.240)224208Branch 5 12 hosts /28 (255.255.255.240)WAN 5


*Applying the Addresses to the Topology


CLASSLESS INTERDOMAIN ROUTINGCIDR (pronounced cider) ignores address class. With CIDR, a router use a bit mask to determine the network and host portions of an address. CIDR replaced the categories (A, B, C, etc.) with a more generalized network prefix. This prefix could be of any length rather than just 8, 16, or 24 bits. This allows CIDR to craft network address spaces according to the size of a network instead of force-fitting networks into presized network address spaces.*


CLASSLESS INTERDOMAIN ROUTINGCIDR sounds a lot like VLSMCIDR is usually discussed in general Internet context (ISPs)Uses custom length prefixes to reduce workload in key Internet routersVLSM is usually discussed in enterprise contextUses custom length prefixes to have better usage of enterprise address space*


CLASSLESS INTERDOMAIN ROUTINGRouters use the network-prefix, rather than the first 3 bits of the IP address, to determine the dividing point between the network number and the host number. In the CIDR model, each piece of routing information is advertised with a bit mask or prefix-length ( /x ). The prefix-length is a way of specifying the number of leftmost contiguous bits in the network-portion of each routing table entry.*


CLASSLESS INTERDOMAIN ROUTINGFor example, a network with 20 bits of network-number and 12 bits of host-number would be advertised with a 20 bit prefix (/20). The clever thing is that the IP address advertised with the /20 prefix could be a former Class A, Class B, or Class C. All addresses with a /20 prefix represent the same amount of address space (212 or 4,096 host addresses).20 bits network + 12 bits host

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CLASSLESS INTERDOMAIN ROUTINGInstead of handing out an entire A, B, or C network to an organization, address space can be assigned in chunks that fit the need.If an organization needs 254 host addresses, what difference does it make whether they are given: a Class C (200.23.76.0 /24)1/256th of a Class B (145.38.20.0 /24)1/65,536th of a Class A (91.187.7.0 /24)Using a /24 prefix, each of these specifies eight host bits which will support 254 hosts.Note: Each former Class C can be referred to as a /24.*


*

Class

The Class System

Address ClassApplicationNumber of Network BitsNumber of Host BitsDecimal Address RangeNumber of AddressesNumber of Possible Host

Class ALarge Networks8 bits24 bits1 - 12612616,777,214

Class BMedium-sized Networks16 bits16 bits128 - 19165000 +65,534

Class CSmall Networks24 bits8 bits192 - 223Over 2 Million254

Prefix

Prefix Equivalents

Network PrefixEquivalent Number of Class AddressesNumber of Hosts



/251/2 of a Class C128

/241 Class C or 1 /24256

/232 Class C or 2 /24s512

/224 Class C or 4 /24s1,024

/218 Class C or 8 /24s2,048

/2016 Class C or 16 /24s4,096

/1932 Class C or 32 /24s8,192

/1864 Class C or 64 /24s16,384

/17128 Class C or 128 /24s32,768



/141,024 Class C or 4 Class B262,144


/124096 Class C or 16 Class B1,048,576

/118192 Class C or 32 Class B2,097,152

/1016384 Class C or 64 Class B4,194,304

/932768 Class C or 128 Class B8,388,608

/865,536 Class C or 256 Class B or 1 Class A16,777,216

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day 5.5 subnetting.ppt

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